I was playing for a chorus concert on an electric keyboard, of course. The sustain pedal went out on the keyboard during the middle of the performance and I had to survive the entire rest of the concert without the pedal. Talk about a lot of finger exchanges and octave changes on the spot!
One time I was playing for a gig outside and a bee landed on my music while I was performing. Terrified and allergic to bees, I had to think fast. So all I could think of was to turn my page quickly and smash the bee between the pages. It worked! (And no one ever knew it happened!)
I was playing two recitals in one day. The first was a student recital that was scheduled for 4:30 that afternoon. The second was an off-campus event unaffiliated with the school and was scheduled for 6:00 that night. I planned to finish the first recital and head to the second recital since venue was only five minutes away. That morning, I received an email from the organizer of the 6:00 recital with the subject "URGENT". He sent the email to let us know that there had been a miscommunication about the start time of the recital and it was actually scheduled to start at 4:00 that afternoon, leaving a 30-minute overlap between the two recitals. After reading the email, I spent the next few hours frantically making phone calls to see what could be done and we finally came to a solution. I would start the (formerly 6:00) voice recital and leave at 4:15. The students' teacher (who was also a fantastic pianist) took over for me while I rushed to school to play the student recital!