Had a few of these moments, in no particular orders small selection:
forgetting to unmute my amp after tuning so when it was time for my big bass entry in the song, nothing happened;
the drummer misunderstanding the direction from the band leader to play quieter during my flute solo - he thought he was being instructed to slow down so all the energy went from the music as everyone else then lost their place in the form;
battery dying in an active bass partway through a song;
setting up at the gig only to realise we had left the piano stand on the pavement outside the drummer’s house;
meeting at a major festival where we had the headline slot the next night and finding out the drummer had totally forgotten to bring the piano, which meant I had to ask the headline band of that night whether their piano player might be able to loan us his piano, thankfully he did;
During the sound check for the above gig, finding that the saxophonist’s instrument needed a quite significant repair in order to function and emergency surgery was performed by the drummer;
during a workshop before a gig the same night, seeing the relief on my fretless getting lower and lower until it became unplayable due to the heat;
having the flute player’s frustrated exhortation of “no fùcking drums!” get broadcast at full volume over the PA when the drummer forgot the arrangement again;
drunken party guest falling into the stage monitors mid-song, shearing the cable and knocking over a glass of wine and seeing the liquid flowing towards the exposed live pins of the cable in front of me.