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Error of the Day

• Comprised of: the phrase editors keep circling 'Is comprised of' is common but many editors still flag it. The whole comprises the parts; the parts compose t
• Error of the Day One common error a day, explained and sourced: confused words, myths, cognitive biases, developer errors and m

erroneo.us

• About What erroneo.us is, who publishes it, how pages are researched and updated, and how to report an error on a si
• Sources How erroneo.us researches, cites and checks every page, plus the complete bibliography of specifications, repo

Play

• Bias or Fallacy? quiz A free quiz of 24 everyday scenarios. Can you name the cognitive bias or logical fallacy in each? Instant feed
• Error Message Decoder Paste a JavaScript or Python error message and get what it means and how to fix it. Runs entirely in your brow
• Spot the Error A free, timed proofreading game: find the one wrong word in each sentence. Confused words, grammar, idioms and
• Play with your errors Free interactive tools: a proofreading game, a bias-or-fallacy quiz, a JS and Python error decoder, an HTTP st

Developers

• Developer errors Plain-English references for HTTP status codes, common JavaScript and Python error messages, and the famous so
• Famous software bugs in history Ariane 5, Mars Climate Orbiter, Therac-25, Knight Capital, the Patriot clock, Y2K and CrowdStrike: what went w
• 401 Unauthorized vs 403 Forbidden 401 means authenticate and try again; 403 means credentials will not help. The RFC 9110 rules, OAuth bearer-to
• 404 Not Found What HTTP 404 Not Found really means under RFC 9110, how it differs from 410 Gone, why soft 404s hurt SEO, and
• 418 I'm a teapot Where HTTP 418 I'm a teapot came from (RFC 2324, April 1998), why RFC 9110 reserves it as (Unused), and whethe
• 429 Too Many Requests HTTP 429 means you hit a rate limit. What RFC 6585 says, how to honor Retry-After, exponential backoff with ji
• 500 Internal Server Error HTTP 500 means the server hit an unexpected condition. How to find the real cause in logs, the usual culprits,
• 502 Bad Gateway vs 504 Gateway Timeout (and 503) 502, 503 and 504 all mean something behind the proxy failed. How to tell them apart, read nginx, Caddy and loa
• HTTP status codes, explained properly Every HTTP status code from 100 to 511 in plain English: what each means per RFC 9110, the usual cause, and th
• Common JavaScript errors, explained What "Cannot read properties of undefined", "x is not a function", CORS and JSON.parse errors mean, with exact
• Common Python errors, explained IndentationError, NameError, 'NoneType' object is not subscriptable, KeyError, ModuleNotFoundError and more: w

Language

• Language errors Guides to the English mistakes people search for most: commonly confused words, grammar errors, misspellings,
• Common grammar mistakes The grammar mistakes that actually cost you credibility, from comma splices to 'between you and I', plus the f
• Commonly confused words Affect or effect? Then or than? A skimmable table of 45 commonly confused English words with meanings, example
• Commonly mispronounced words Nuclear, espresso, February, often, GIF: which pronunciations dictionaries call nonstandard, which are fine, a
• Commonly misspelled words Separate, definitely, accommodate, embarrass: a table of 50 commonly misspelled English words with the usual w
• Misused idioms and phrases For all intents and purposes, couldn't care less, begs the question, irregardless, literally: the real forms,

Myths

• Myths & misconceptions Popular myths about science, history, nutrition and money checked against primary sources: NASA, the FDA, the
• History myths: what really happened Was Napoleon short? Did Vikings wear horned helmets? Did Marie Antoinette say 'let them eat cake'? 11 popular
• Money mistakes and personal-finance myths Minimum payments, credit-check myths, skipped 401(k) matches, fee drag, market timing, forgotten subscriptions
• Nutrition myths: what the evidence actually says Eggs and cholesterol, MSG, detoxes, eight glasses of water, the five-second rule, best-by dates: 13 food myths
• Science myths that refuse to die Do we use 10% of our brains? Is the Great Wall visible from space? Is glass a liquid? 13 common science myths,

Thinking

• Thinking errors Cognitive biases and logical fallacies explained with everyday examples, the research behind them, and practic
• Cognitive biases: the predictable ways good minds go wrong A sourced guide to 20 common cognitive biases: anchoring, confirmation bias, sunk cost, Dunning-Kruger and mor
• Logical fallacies: how arguments break, with examples Straw man, ad hominem, false dilemma, slippery slope, post hoc and more: 20 logical fallacies explained with p