I've posted this elsewhere, but the one when the vocalist had spent the previous few days smoking a large bag of weed. Not eating, or attending to personal hygiene or anything, just smoking the weed.
Come the day of the gig, someone had to go get him as he hadn't turned up.
He shambled on stage and instead of singing, he just made squealing and moaning sounds. The very much shortened first set was ended when his mother dragged him off stage, and drove him to a mental health facility.