I've also been amazed at seeing people obviously enjoy what I was doing while mentally feeling that I had made some serious mistakes. I think a recording is less forgiving than a live performance, however, because people have time to "analyze" a recording. Also, as McGrath pointed out, at a live performance they're experiencing more than just the music - it's the venue, the atmosphere, your "charisma" (assuming that if you perform you must at least have some!). I try to eliminate mistakes by practicing, but I'd rather make a few mistakes and put feeling into the music than be technically perfect. There's nothing worse than music that's "cold". Worst mistakes that I really dread making - starting in the wrong key and that horrible moment when I realize I won't be able to hit the notes, or the total blank out of a whole verse.