Yeah, I've seen some shit fuck ones.
Worst that I recall was David Allen Coe. $85 tickets, mother fucker comes out, plays 45sec from 13/14 songs as a medley, and cut bait. The house band that did the warm-up played for 1hr 15m. Coe came on about a minute after 9pm, and at 9:15 was nowhere to be seen.
Saw Saliva/Audioslave/Theory of a Deadman. Theory was really good, clean set. Audioslave was so fucking bad my buddy and I turned around and had a conversation. A couple times between songs I told the mix guys I was sorry they had to try and work with that.
Was at the show Faster Pussycat got booed off stage and didn't return. They sucked and were playing like shit. Can't remember the whole bill, but they were first, and LA Guns (IIRC) after that. Can't remember if it was a triple or LA was headlining.
I've seen Ice Nine Kills, or however you spell it, a couple times starting off a Metallica show. Not a fan. They do/did some weird sort of performing art shit show that I just didn't get.
Seen various guys drunker than shit on occasion, been in shows that were hotter than Hades ('85 Tx June Jam and $20 waters, anyone?) and "shit" like that too.
I'll think on it a bit more, but the first two stick out the most in terms of bad performance.