I haven't had as many opportunities as some here have had yet, but they're coming!
However, I remember a couple of years ago when I was performing Queen Elizabeth's Galliard by John Dowland up at Shoreline Communitty college in Seattle. Everything was going great until I forgot how the piece was suppose to end. I made something up but don't remember it as being to convincing.
Last year, my choir took a short tour (to Olympia, we could of just driven home instead of spend the night down there and still made the next day's gigs). The first stop was at a highschool south of Tacoma, performing for their choir. The men were to sing an arrangement of Copland's The Boatman's Dance. However, the music arrived late and we didn't have much time to learn it. After a pause of silence, the conducter started signaling the huge forte entrance. However, when gave us the entrance, no one came in. I thought his disappointed look was funny, which got rest of the guys to start laughing. From there, the girls, then highschoolers all joined in. When we got back on the bus, The director announces "we are not going to do the Boatman's dance the rest of the tour, when we get back, that's the first thing we are going to work on."
At the performance after we got back, I missed a dramatic cutoff in the middle of Oh Freedom which earned me a few stares.