Had the singer walk off stage and out the door halfway through a gig after an arguement with some punters while the rest of us were... well... let's say somewhat sedated and not fully capable of grasping entirely what was going on and so carried on regardless. That was pretty exciting, but it was the early 90s and we were just kids, things were different then.
More recently, played a gig where not only could the drummer not hear the vocals in the monitor but the mic lead was dodgy and the singer didn't address it, just tried to hold it in position so it worked rather than utilise one of the BV mics which were already on stage. We have a song with where the vocals are the cue back in after dead stops, very powerful and dynamic when it works, utterly embarassing when the mic lead craps out during both of them and the drummer, who can't watch the singer's mouth, comes in about half a second after the bass and guitar. What was already a truly crap experience became a genuine ground-please-open-up moment.