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😱 Falsehoods Programmers Believe in

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Aug 16, 2025

Business

• “The system can’t handle a billion dollars” - Insane compensation at Meta in the middle of the AI-boom breaks ERPs.

Jun 19, 2025

Geography

• Falsehoods about Maps - Covers coordinates, projection and GIS.

• Falsehoods about Weather - Weather is location-dependent, and so full of edge-cases.

Jun 08, 2025

Business

• Decimal Point Error in Etsy's Accounting System - The importance of types in accounting software: missing the decimal point ends up with 100x over-charges.

• Twenty five thousand dollars of funny money - Same error as above at Google Ads, or the danger of separating your pennies from your dollars, where $250 internal coupons turned into $25,000. My advice: get rid of integers and floats for monetary values. Use decimals. Or fallback to strings and parse them, don't validate.

Dates and Time

• ISO-8601, YYYY, yyyy, and why your year may be wrong - String formatting of date is hard.

• Why is subtracting these two times (in 1927) giving a strange result? - Infamous Stack Overflow answer about both complicated historical timezones, and how historical dates can be re-interpreted by newer versions of software.

Networks

• Falsehoods about Networks - Covers TCP, DHCP, DNS, VLANs and IPv4/v6.

Transportation

• My name causes an issue with any booking! - Old airline reservation systems considers the MR suffix as Mister and drops it.

Jun 07, 2025

Transportation

• Falsehoods about Aviation - Aviation data are less normalized than you might think.

Jun 06, 2025

Software Engineering

• Falsehoods about authorization - Misconceptions about implementing permissions systems.

Apr 21, 2025

Software Engineering

• Falsehoods about CVE - CVE ≠ vulnerability (and 36 other confusions).

Mar 04, 2025

Internationalization

• Falsehoods about Language - Additional cases to complement the previous article.

Feb 13, 2025

Software Engineering

• Myths about CPU Caches - Misconceptions about caches often lead to false assertions, especially when it comes to concurrency and race conditions.

• Falsehoods about null pointers - Null pointers are even more cursed than pointers in general, and provenance already makes pointers quite complicated.

• Falsehoods about Digital Object Identifiers (DOIs) - False conceptions about the identifiers that are used to identify and link research outputs (and a lot of other things).

Nov 03, 2024

Software Engineering

• Facts about State Machines - State machines are often misunderstood and under-applied.

Jul 05, 2024

Internationalization

• Unicode misconceptions - A collection of falsehoods on case, encodings, string length, and more.

Jul 01, 2024

Software Engineering

• Falsehoods about Event-Driven Systems - Misconceptions about event driven systems and message passing.

Jun 04, 2024

Meta

• Falsehoods about Falsehoods Lists - Meta commentary on how these falsehoods shouldn't be handled.

Apr 16, 2024

Web

• Falsehoods about HTML - “Web is beautiful. Web is ugly. Web is astonishing. A part of this appeal is HTML, with its historical quirks.”

• The Hidden Complexity of Downloading Favicons, Told in 15+ Edge Cases - Downloading that little icon you see in you browser tabs should be a simple exercise. It turned out to be a lot more complicated than you think. Be vigilant that you are not shaving a Yak.

Mar 19, 2024

Emails

• libvldmail - A library that implements RFC-based checks for e-mail addresses.

Mar 04, 2024

Software Engineering

• I am endlessly fascinated with content tagging systems - There are edge-cases even in tagging systems which are supposed to be barebone.

Nov 08, 2023

Internationalization

• Localization Failure: Temperature is Hard - You cannot localize temperature differences as-is.

Postal Addresses

• UK Address Oddities - Quirks extracted from a list of most residential property sales in England and Wales since 1995.

Jun 14, 2023

Geography

• Top 5 most insane kanji place names in Japan - “There's one special group of kanji that's hard even for Japanese people to read: place names.”

Jun 10, 2023

Dates and Time

• “I'm going to a commune in Vermont and will deal with no unit of time shorter than a season.” - Is the note left on his terminal by a quitting engineer in the 70s, after too much effort toiling away on sub-second timing concerns. Source: The Soul of a New Machine.

Apr 21, 2023

Software Engineering

• The weird world of Windows file paths - “On any Unix-derived system, a path is an admirably simple thing: if it starts with a /, it's a path. Not so on Windows.”

Mar 07, 2023

Postal Addresses

• Why doesn't Costa Rica use real addresses? - Costa Rican uses an idiosyncratic system of addresses that relies on landmarks, history and quite a bit of guesswork.

• USPS Postal Addressing Standards - Describes both standardized address formats and content.

Feb 02, 2023

Cryptocurrency

• Falsehoods about Bitcoin (⭐78) - A list of mistaken perspectives on Bitcoin.

• Falsehoods about Ethereum - Misconceptions and common pitfalls in contract programming.

Jan 12, 2023

Software Engineering

• The yaml document from hell - YAML is full of obscure complexity like accidental numbers and non-string keys.

Dec 26, 2022

Software Engineering

• Falsehoods about Quantum Technology (⭐11) - Common misconceptions about quantum technology and computers.

Dec 19, 2022

Networks

• IDN is crazy - International characters in domain names mean support of homographs and heterographs.

Dec 14, 2022

Transportation

• Falsehoods about Cars (⭐6) - Even something as common as defining a car is full of pitfalls.

• Falsehoods about Airline Seat Maps - Airline seat maps are far more complex than just neat rows and columns of seats.

• The Maddening Mess of Airport Codes - Having multiple international and national agencies trying to reconcile history, practicality and logistics makes codes follow arcane rules.

Web

• Falsehoods about REST APIs - Pitfalls to be mindful of when creating and documenting APIs.

• URLs: It's complicated… - There's a lot of components in an URL, and all have their own logic.

Dec 07, 2022

Software Engineering

• Falsehoods about Undefined Behavior - Invoking undefined behavior can cause anything to happen, for a much broader definition of "anything" than one might think.

Dec 03, 2022

Business

• Minutiae of company names - How the rules of the State of Delaware and the IRS does not intersects.

Dates and Time

• Storing UTC is not a silver bullet - “Just store dates in UTC” is not always the right approach.

Education

• Postdoc myths - “Lots of things are said, written and believed about postdoctoral researchers that are simply not true.”

Software Engineering

• Hi! My name is… - This talk could have been named falsehoods about usernames (and other identifiers).

• Floating Point Math - “Your language isn't broken, it's doing floating point math. (…) This is why, more often than not, 0.1 + 0.2 != 0.3.”

Sep 01, 2022

Emails

• Falsehoods about Email - On addresses, content and delivery.

Jun 21, 2022

Business

• Falsehoods about Online Shopping - Covers prices, currencies and inventory.

Geography

• Falsehoods about Geography - Takes on places, their names and locations.

Internationalization

• Falsehoods about Plain Text - Plain text can't cut it, which makes Unicode even more incredible for its ability to just work well.

• Falsehoods about text - A subset of the falsehoods from above, illustrated with some examples.

Apr 26, 2022

Dates and Time

• The Long, Painful History of Time - Most of the idiosyncrasies in timekeeping can find an explanation in history.

• How to choose between UT1, TAI and UTC - Depends on your priorities between SI seconds, earth rotation sync, leap seconds avoidance.

• Critical and Significant Dates - From Y2K to the overflow of 32-bit seconds from Unix epoch, a list of special date to watch for depending on the system.

Software Engineering

• Rules for Autocomplete - Not falsehoods per se, but still a great list of good practices to implement autocompletion.

Apr 04, 2022

Multimedia

• Horrible edge cases to consider when dealing with music - Music catalogs data are full of crazy stuff.

• MusicBrainz database schema - An open-source project and database that seems to have solved the complexity of music catalog management.

• DDEX - The industry standard for music metadata, including archiving, sound recording, sales and usage reporting, royalties and license deals.

• Apple Music Style Guide - Quality insurance guidelines to format music, art, and metadata to increase discoverability.

Mar 30, 2022

Dates and Time

• Falsehoods about Unix Time - Mind the leap second!

• $26,000 Overcollection by Labor Department - The consequence of wrong calendar accounting.

Human Identity

• Falsehoods about Gender: #1 & #2 - Gender is part of human identity and has its own subtleties.

• XKCD #327: Exploits of a Mom - Funny take on how implementation of a falsehood might lead to security holes.

Phone Numbers

• libphonenumber - Google's common Java, C++ and JavaScript library for parsing, formatting, and validating international phone numbers. Also available for C# (⭐855), Objective-C (⭐2.4k), Python (⭐3.6k), Ruby (⭐548) and PHP (⭐4.9k).

Jan 11, 2022

Business

• CLDR currency definitions (⭐1k) - Currency validity date ranges overlap due to revolts, invasions, new constitutions, and slow planned adoption.

Oct 24, 2021

Dates and Time

• RFC-3339 vs ISO-8601 - An giant list of formats from the two standards, how they overlaps, and live examples.

Oct 18, 2021

Software Engineering

• What every software engineer should know about search - A better sourced article on the difficulty of implementing search engines.

Oct 08, 2021

Software Engineering

• Falsehoods about Package Managers - Covers package and their managers.

Sep 06, 2021

Video Games

• The Door Problem - All the things you have not considered implementing for your doors in games.

Sep 02, 2021

Emails

• Your E-Mail Validation Logic is Wrong - A summary of the various, surprising things that are allowed in an email address.

Geography

• I Hate Coordinate Systems - A guide for geospatial practitioners on diagnosing and fixing common issues with coordinate systems.

Software Engineering

• Falsehoods about garbage collection - Misconceptions about the predictability and performance of garbage collection.

Feb 08, 2021

Meta

• Falsehoods about Programming - A humbling and fun list on programming and programmers themselves.

Arts

• Falsehoods about Music - False assumption that might be made in codifying music.

• Falsehoods about Art - Common misconceptions about art.

Business

• Falsehoods about Prices - Covers currencies, amounts and localization.

• Falsehoods about IBANs (⭐493) - International Bank Account Numbers are not international.

• Falsehoods about Economics - Economics are not simple or rational.

Dates and Time

• Falsehoods about Time - Seminal article on dates and time.

• More Falsehoods about Time - Part. 2 of the article above.

• Falsehoods about Time and Time Zones - Another takes on time-related falsehoods, with an emphasis on time zones.

• Critique of Falsehoods about Time - Takes on the first article above and provides an explanation of each falsehood, with more context and external resources.

• Falsehoods about Time Zones - Has some nice points regarding the edge-cases of DST transitions.

Human Identity

• Falsehoods about Names - The article that started it all.

• Falsehoods about Names – With Examples - A revisited version of the article above, this time with detailed explanations.

• Falsehoods about Biometrics - Fingerprints are not unique.

• Falsehoods about Families - You can't really define a family with strict rules.

• Falsehoods about Me - Issues at the intersection of names and gender and internationalization.

Internationalization

• Falsehoods about Language - Translating a software from English is not as straightforward as it seems to be.

Management

• Falsehoods about Job Applicants - Assumptions about job applicants and their job histories aren't necessarily true.

Multimedia

• Falsehoods about Video - Cover it all: video decoding and playback, files, image scaling, color spaces and conversion, displays and subtitles.

Phone Numbers

• Falsehoods about Phone Numbers (⭐17k) - Covers phone numbers, their representation and meaning.

Postal Addresses

• Falsehoods about Addresses - Covers streets, postal codes, buildings, cities and countries.

• Falsehoods about Residence - It's not only about the address itself, but the relationship between a person and its residence.

Science

• Falsehoods about Systems of Measurement - On working with systems of measurement and converting between them.

Society

• Falsehoods about Political Appointments - Designing election systems has its own tricks.

• Falsehoods about Women In Tech - Myth about women in STEM (Science, Technology, Engineering, Math) industries.

Software Engineering

• Falsehoods about Versions (⭐68) - Attributing an identity to a software release might be harder than thought.

• Falsehoods about Build Systems - Building software is hard. Building software that builds software is harder.

• Falsehoods about CSVs - While RFC4180 to exists, it is far from definitive and goes largely ignored.

• Falsehoods about Testing - An attempt to establish a list of falsehoods about testing.

• Falsehoods about Search - Why search (including analysis, tokenization, highlighting) is deceptively complex.

• Falsehoods about Pagination - Why your pagination algorithm is giving someone (possibly you) a headache.

• Myths about File Paths - Diversity of file-systems and OSes makes file paths a little harder than we might think of.

• Myths about /dev/urandom - There are a few things about /dev/urandom and /dev/random that are repeated again and again. Still they are false.

Typography

• Falsehoods about Fonts (⭐105) - Assumptions about typography on the web and in desktop applications.

Dec 07, 2020

Meta

• Falsehoods Programmers Believe - A brief list of common falsehoods. A great overview and quick introduction into the world of falsehoods.

Business

• tax - A PHP 5.4+ tax management library.

Dates and Time

• Your Calendrical Fallacy Is Thinking… - List covering intercalation and cultural influence, made by a community of iOS and macOS developers.

• Time Zone Database - Code and data that represent the history of local time for many representative locations around the globe.

• You Advocate a Calendar Reform - Your idea will not work. This article tells you why.

• So You Want to Abolish Time Zones - Abolishing timezones may sound like a good idea, but there are quite a few complications that make it not quite so.

• The Problem with Time & Timezones - A video about why you should never, ever deal with timezones if you can help it.

• UTC is Enough for everyone, right? - There are edge cases about dates and time (specifically UTC) that you probably haven't thought of.

Education

• Falsehoods CS Students (Still) Believe Upon Graduating - A list of things (not only) computer science students tend to erroneously and at times surprisingly believe even though they (probably) should know better.

Emails

• I Knew How to Validate an Email Address Until I Read the RFC - Provides intricate examples that are unsuspected valid email addresses according the RFC-822.

• So you think you can validate email addresses (FOSDEM 2018) - Presentation of edge-case email addresses and why you should not use regex to parse them.

Human Identity

• Gay Marriage: The Database Engineering Perspective - How to store a marriage in a database while addressing most of the falsehoods about gender, naming and relationships.

• Personal Names Around the World - How do people's names differ around the world, and what are the implications for the Web?

• Hello, I'm Mr. Null. My Name Makes Me Invisible to Computers - Real-life example on how implemented falsehood has negative impact on someone's life.

• HL7 v3 RIM - A flexible data model for representing human names.

• Apple iOS NSPersonNameComponentsFormatter - Localized representations of the components of a person's name.

Internationalization

• Internationalis(z)ing Code - A video about things you need to keep in mind when internationalizing your code.

• Minimum to Know About Unicode and Character Sets - A good introduction to unicode, its historical context and origins, followed by an overview of its inner working.

• Awesome Unicode (⭐942) - A curated list of delightful Unicode tidbits, packages and resources.

• Dark corners of Unicode - Unicode is extensive, here be dragons.

• Let's Stop Ascribing Meaning to Code Points - Dives deeper in Unicode and dispels myths about code points.

• Breaking Our Latin-1 Assumptions - Most programmers spend so much time with Latin-1 they forgets about other's scripts quirks.

• Ode to a shipping label - Character encoding is hard, more so when each broken layer of data input adds its own spice.

• i18n Testing Data (⭐65) - Compilation of real-word international and diverse name data for unit testing and QA.

• Big List of Naughty Strings (⭐47k) - A huge corpus of strings which have a high probability of causing issues when used as user-input data. A must have set of practical edge-cases to test your software against.

Networks

• Fallacies of Distributed Computing - Assumptions that programmers new to distributed applications invariably make.

• There's more than one way to write an IP address - Some parts of the address are optional, mind the decimal and octal notations, and don't forget IPv6 either.

• hostname-validate - An attempt to validate hostnames in Python.

Postal Addresses

• Letter Delivered Despite No Name, No Address - Ultimate falsehood about postal addresses: you do not need one.

• The Bear with Its Own ZIP Code - Smokey Bear has his own ZIP Code (20252) because he gets so much mail.

• Regex and Postal Addresses - Why regular expressions and street addresses do not mix.

• libaddressinput - Google's common C++ and Java library for parsing, formatting, and validating international postal addresses.

• addressing - A PHP 5.4+ addressing library, powered by Google's dataset.

• postal-address - Python module to parse, normalize and render postal addresses.

• address - Go library to validate and format addresses using Google's dataset.

Software Engineering

• Popular misconceptions about mtime - Part of a post on why file's mtime comparison could be considered harmful.

Typography

• Truths programmers should know about case - A complete reverse of the falsehoods format, on the topic of case (as in uppercase and lowercase text).

Nov 11, 2020

Postal Addresses

• Parsing the Infamous Japanese Postal CSV - “I saw many horrors, but I've never seen this particular formatting choice anywhere else.”

Oct 28, 2020

Business

• Characters < and > in company names lead to XSS attacks - Because UK allows companies to be registered with special characters, a hacker leveraged them to register \"><SCRIPT SRC=MJT.XSS.HT></SCRIPT> LTD, but also ; DROP TABLE "COMPANIES";-- LTD, BETTS & TWINE LTD and SAFDASD & SFSAF \' SFDAASF\" LTD.