My pick is "A man's a man for a' that". On my own, I can generally sing it without screwing up (although, I have to admit, I've never tried performing it.) However, when I sing it with the quartette I'm in, we never seem to get it right during practices. We all love the song, and are seriously committed to learning the damn thing, but for some reason, the lyrics are illusive when we start to sing it. We have performed it maybe twice--getting it right each time! Go figure!