It's not only instrumentalists that are guilty of joining in inappropriately.

Sometimes when you are singing a song, you can hear a few people round the room singing along with you in barely audible voices. Most of the time, it doesn't really bother me, but sometimes one of them is singing a different version from you and that is annoying.

Choruses are for joining in, but I find sometimes that the rest of the room will join in a chorus, but at a different (usually slower) tempo than the tempo you are singing the song and that can be irritating.