This is a completely awesome topic ... I bow to you, Sir Phill !
I have four crap-shows in mind:
a) Joe Cocker (Halifax, mid-70's)
He was so drunk that he barely got through 3 or 4 songs. He then tried to climb into the back of his piano, and a couple of roadies had to free him from his predicament. The concert came to a close, and we were all eventually refunded for our tickets. He acted like a pompous idiot during his 10-minutes on stage, and I have no respect for him as a result.
b) Seals & Crofts / Nazareth (Halifax, mid-70's)
What kind of promoter books S&C to open for Nazareth? Halfway through their 3rd song, the audience booed them off the stage, and Nazareth came on for a terrific performance. It was a weird experience.
c) April Wine (Montreal, mid-70's)
The sound system blended everything into a wall of noise, with no one instrument being discernible from the rest. I give the sound people 0/10 for their part. When lead-singer Myles Goodwin stepped to the front of the stage, he played the worst guitar solo I've ever heard by anyone claiming to be a professional. Missed notes, dropped pick twice, and just plain dreadful. We were about 15-feet away from him, and we all booed him at the top of our lungs. He cut it short, and we all cheered. Pretty cruel, but he should have left the lead guitar work to Gary Moffatt (who was quite good).
d) Rockin' the Island (Baddeck, Cape Breton, mid-70's)
An outdoor weekend concert with 6 or 8 decent bands was advertised, and we bought tickets and went. We got there, saw the stage, lighting, PA system all set up, so we pitched our tents along with hundreds of others. No bands ever showed up, and no explanation was ever given. A bunch of people trashed the stage setup, but we just left.