When I tried to sign up for MusicBrainz with a password that's too long (128 characters - 64 worked), the error message was rather cryptic and didn't reveal the maximum allowed password length:

Field should not exceed [quant,_1,character]. You entered [_2]

This seems to have worked correctly in the past (for example, this comment quotes an error message with working character counts), but presumably broke at some point.