Got a couple good ones:
*Once when I was really little, I about to play some Handel sonatina (A minor?) at a recital, but the piano bench was this old wooden rickity thing, and as I was adjusting it, I got a splinter in one of my fingers. It was awful, but I didn't let on and played my piece. Painful. Every time I pressed the key with that finger, a little stab of pain hit me.
*When I was in high school, a fellow pianist and I were performing some 4-hand stuff at a recital. For whatever reason, we didn't have a page-turner, and during our performance, the pages of our music kept slowing flipping back to the beginning of the book. The piece was so busy that neither of us could afford to give up a hand to hold the music open, and no one in the audience came up to help us, so for the next 12 minutes or so, both of us frantically used our lung power to hold the correct pages open by blowing on them!
*In college, I was accompaning a tenor who was to sing this slow, languid, solemn song about slavery at an African-American Festival concert. We'd been rehearsing for weeks, and it was just lovely. Well, night of the concert, I'm back stage waiting for him to arrive so that we can warm up. My tenor arrives wearing this white zoot suit, complete with a white fedora, spats, and the pocket-watch chain. ?? Umm, OK. Warm up went fine. Then, it's our turn to perform.
We get out on stage, and after all the hooting and whistles from the women in the audience (in appreciation for his dapper dress), we begin the piece. Well, he completely launches into this hammed-up, Big-Bad-Voodoo-Daddy-esque, Jump-Jivin' Scat version of the song!! I almost had a heart attack! Of course, he's dancing the Lindy-Hop and the Jitterbug up there, and the audience is really going NUTS, and I'm sitting there at the piano in a formal black dress, trying my best to ham up my piano part to keep up with him, wishing the piano bench came with a seatbelt... Ugggh!