People rely on undocumented behavior all the time. In fact, most projects would be unusable if you didn't – it's a rare project that's perfectly documented. You

People rely on undocumented behavior all the time. In fact, most projects would be unusable if you didn't – it's a rare project that's perfectly documented. You're not going to hear from most of your users when you break stuff for them – most of them will just be silently annoyed. Enough annoyance from enough people and your library/language/whatever develops a bad reputation.