My biggest bugbear is when a singer chants a verse and a half of a song, gets stuck then announces that they'd be better singing another song. They then proceed to waste another 5 minutes of our lives digging through their binder(s) before treating us to yet another classic.
Just as bad is when they get half way through a song, screw up and say "I'll start again".

By that time, I've usually lost the will to live.