My two main peeves:

1. Talking through someone else's song.
2. Bringing a song one is totally unprepared to sing. Forgetting a verse or two, stumbling a few times - that's a different story, and I believe in patience and encouragement when that happens, but subjecting everyone to a song one is completely unprepared to sing, that's another kettle of fish. What's the point?

Chanteyranger