The biggest mistake I ever made during a live performance was when I literally fell asleep on stage.
It was the summer of 2001 (I was 9 years old) and I was playing the role of Michael Darling in an SRT production of Peter Pan. It was a super fun summer -- I got to stunt fly in every performance! The only downside was that I had to wear an uncomfortable harness all the time.
This particular performance was the third show of the day... I was really tired. It was a full house. We were doing the scene where Wendy first meets Peter, right before everyone flies to Neverland. In the scene, the two younger brothers are "sleeping" while Wendy and Peter talk.
Despite the uncomfortable harness, scratchy blanket and bright stage lights, I somehow fell asleep on stage.
Peter Pan said my cue.
No response.
She tried again, but I still didn't respond.
At this point, Peter Pan realized the situation and kicked me HARD. I leapt out of bed and said my line in character.
The audience had no idea what happened, but the actors never stopped giving me shit for that one!