White Zombie, early 90's in support of Astro Creep. Bumped into a couple buddies; one a life-long stoner jackass and the other a good dude that got completely shitfaced. Stoner bud says "Hey man, Matt's pretty fucked up, will you keep an eye on him for a minute?". My wife and I, being cool people, said "sure". Stoner John flat out disappeared the rest of the night, leaving us with Matt, whose condition got worse. We ended up spending the entire show in a medical tent with him, while he was hooked up to ekg monitors, etc. Missed the entire thing. Should have just left the dumbass, but we did right by him. That was pretty much the last time I ever talked to John (final straw with his stupidity) and I did get the satisfaction of knowing that Matt ended up beating the shit out of John sometime later for an unrelated stoner fuckup incident.
Runner-up would be Bulletboys, maybe 10 years ago. Saw they were coming to a local place and was like "Oh cool...Smooth up in ya". Went and including our group of 4, there might have been 25 people in the entire place, including the wait staff, band and their crew. At that point in his music career it was pretty much the singer, Marc something and a bunch of baby-faced hired gun musicians filling in. He'd yell "Raise your haaaaaaannnnnndddsss!!!" and 2 arms would go up, not including the crickets. Never felt so awkwardly bad for a musician. Just kept thinking, at some point this dude was playing stadiums and now he's in some SW PA shithole that has plastic palm trees and 5 people in it.