Had a friend's birthday party, she booked the town hall, and had a few friends' bands lined up to play on the stage, I was merely in attendance. My good friend's band's bass player no showed, I was thrust a bass (already semi-pissed) and asked to go on stage, to play a bunch of covers I didn't know, a large amount of guesswork and copying the guitarists hand movements ensued, they also tried happy birthday, that was bad, I do not know how to play happy birthday. It got worse after we'd mucked through 4 songs and they were like, next song is this one....guitarist, who was new also, thrusts me the guitar and says, I don't know this one, I muck along to Pinball Map by In Flames, solo and all, not knowing a single note in the song. Awful experience, but pulled the birthday girl, jackpot!
Had a bad one at a gig I organised, got a touring band coming over from holland for it along with a few others from all over the uk. I thought my a string was feeling dodgy in practice so changed it last minute. 30s into a very e-oriented song of around 5 minutes, my e-string goes, oh the irony. With everyone else in completely miles off tunings, I run off stage post song, rip a string off the previous band's bass and replace mine, the band wait a coupla minutes, but then crack on with the next song. I'm part way through winding my string when the solo comes. This solo has a very high bass solo, with the one guitar harmonised with it. There was no bass, there was my guitarist playing a harmony part alone, sounded dreadful. I emerged for the rest of the set, which was fine, I just wish they had thought not to play that song of all of them, without me!