Joined a band. At a gig the singer called out a number and the band played it. Thing is, I hadn't learnt it - because I'd never heard it before, we hadn't rehearsed it, it had never even been previously discussed or put forward. To say I was a bit annoyed is an understatement. I should have walked there and then, because as it turned out this was symptomatic of the singer's attitude towards his band, i.e. no consideration whatsoever given to anyone. It didn't end well, obviously...
I avoided my worst nightmare for 40 years (Trying to play a song I don't know in front of an audience) and this idiot made it a reality!!