Why YAML Turns NO Into false

Write a list of ISO country codes in YAML and one of them behaves differently from the rest:

countries: [GB, FR, DE, NO, SE] — every entry is the string you wrote, except NO, which the parser reads as the boolean false. YAML 1.1 accepts y, yes, n, no, on, off, true and false as booleans, in any capitalisation, and NO for Norway matches one of them.

The failure is quiet, which is what makes it dangerous. Nothing errors; a downstream lookup simply fails to find a country called 'false' and the entry disappears. The same applies to ON for Ontario and to any single letter Y or N used as a code.

Version numbers are the second common trap. `version: 1.2` parses as the number 1.2, while `version: 1.2.3` stays a string, because the second is not a valid number. Two adjacent lines that look identical in style behave differently, and the numeric one loses any trailing zero — 1.10 becomes 1.1, which is a different version entirely.

Leading zeros are a third. A value like 0755 for file permissions may be read as an octal number rather than the string you meant, and a zip code such as 01234 can lose its leading zero and become 1234.

Times are a fourth: `start: 09:30` can be interpreted as a sexagesimal number in YAML 1.1 parsers rather than the string 09:30. Anything that resembles a number, a boolean or a date is a candidate for being reinterpreted.

Quote anything that is meant to be text and could be read as something else. Quoting is never wrong, even where it is unnecessary, so the safe habit is to quote country codes, version numbers, times, anything with a leading zero, and any single-letter value.

`countries: ['GB', 'FR', 'DE', 'NO', 'SE']` behaves exactly as intended. So does `version: '1.2'`.

The other half of the fix is verification: convert the file to JSON and look at what the values actually became. JSON has no type inference, so a string is unambiguously quoted and a number unambiguously is not — which makes any unintended conversion immediately visible rather than something you discover in production.