Jamming RH thumb day before performance, having to play without it.. that was interesting.
Stuck in traffic for 2 hours, freaking out, then running to the stage..
Accompanying a singer, she hands me the music, unbound, and someone opens some door, blowing the music off the stand mid performance, having to improvise the rest.
I attended a friend's performance playing a violin concerto where a string snapped mid performance - and kept on going.
Just a few off the top of my head - a comedy of errors.