erroneo.us
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A reference for common errors · Updated October 2026

Get it wrong right.

A researched, source-linked reference to the mistakes people make most often — in writing, in code, in reasoning, and in what “everybody knows.” Each page shows the wrong version, the right version, and the evidence.

Play Spot the Error →Search the guides ⌘K
  • Could Couldn’t care less
  • Uncaught TypeError
  • 10% of your brain
  • HTTP 418
  • Irregardless Regardless
proofread "I could care less."
✗ could → couldn’t   misused idioms
python3 scores.py
TypeError: 'NoneType' object is not subscriptable
✓ list.sort() returns None. Use sorted().
grep "^15.5.19" rfc9110.txt
15.5.19.  418 (Unused)
✓ reserved in RFC 9110, still not a coffee pot
fact-check "We only use 10% of our brains."
✗ myth   nearly all of it is active over time

Play with your errors

Five interactive tools, built on the same cited guides. Free, no sign-up, nothing leaves your browser.

Spot the ErrorTimed rounds of sentences with one mistake. Tap the wrong word before the red pen does.HTTP Status Code ExplorerEvery registered code, searchable by number, class or keyword, with what to do next.Bias or Fallacy?Everyday scenarios. Name the cognitive bias or logical fallacy hiding inside.Error Message DecoderPaste a JavaScript or Python error. Get the plain-English meaning and the fix.Myth or Fact?Swipe through popular claims. Every verdict is backed by a cited source.

The guides

Four clusters, 21 guides, every claim linked to a source.

Language

Confused words, grammar slips, misspellings, mispronunciations and mangled idioms.

All language guides →

Developers

HTTP status codes, JavaScript and Python errors decoded, and the bugs that made history.

All developers guides →

Thinking

Cognitive biases and logical fallacies: the errors we make before we say a word.

All thinking guides →

Myths

Science, history, nutrition and money misconceptions, checked against primary sources.

All myths guides →

Most looked-up

LanguageCommonly confused wordsAffect or effect? Then or than? A skimmable table of 45 commonly confused English words with meanings, examples and memory tricks that actually stick.DevelopersHTTP status codes, explained properlyEvery HTTP status code from 100 to 511 in plain English: what each means per RFC 9110, the usual cause, and the common mistakes with 401, 404, 429 and 5xx.DevelopersCommon JavaScript errors, explainedWhat "Cannot read properties of undefined", "x is not a function", CORS and JSON.parse errors mean, with exact browser messages and fixes.ThinkingCognitive biases: the predictable ways good minds go wrongA sourced guide to 20 common cognitive biases: anchoring, confirmation bias, sunk cost, Dunning-Kruger and more, with examples, fixes and replication caveats.MythsNutrition myths: what the evidence actually saysEggs and cholesterol, MSG, detoxes, eight glasses of water, the five-second rule, best-by dates: 13 food myths checked against FDA, USDA and peer-reviewed studies.DevelopersFamous software bugs in historyAriane 5, Mars Climate Orbiter, Therac-25, Knight Capital, the Patriot clock, Y2K and CrowdStrike: what went wrong, per official reports, and the lessons.

What this site is#

erroneo.us (say it “erroneous”) is a field guide to being wrong in predictable ways. Some errors are small and social, like writing effect when you meant affect. Some are expensive, like a unit mismatch that sends a spacecraft too deep into a planet’s atmosphere. Most sit in between: a confusing 502 on a Friday deploy, a TypeError that only appears in production, a nutrition rule you absorbed as a child and never checked.

The common thread is that these mistakes are patterned. People confuse the same word pairs, hit the same stack traces and repeat the same myths. That makes them worth documenting once, carefully, with the fix next to the failure.

How the pages work#

  • Wrong next to right. Red strikethrough for the error, green underline for the correction, and a sentence on why.
  • Sources on every claim. Specifications like RFC 9110 for HTTP, official language docs, investigation reports for historical software failures, and government or peer-reviewed sources for health and money. See sources & methodology.
  • Skimmable first, thorough second. Tables and quick answers up top, the nuance underneath, and an FAQ at the end.
  • Corrections welcome. A site named after errors has to be strict about its own. Our corrections policy explains how we fix them.

Start here#

Writers and students usually want commonly confused words or common grammar mistakes. Developers land on the HTTP status code reference, JavaScript errors and Python errors. If you want to argue better, read logical fallacies and cognitive biases. And if you suspect something you were taught is wrong, the myths section is for you.