My worst concert was Huey Lewis and the News. I had seen all the MTV videos where they jumped around and seemed to have a great time on stage. But when I saw them in concert, they all just stood in one place, sang like they didn't want to be there, the sound was muddy as heck, and the warm-up band (can't even remember the name) was better than they were.
It would make an interesting thread to talk about the time you went to a concert to see the main band, and the no-name warmup band went on to become well-known in their own right.
For example, I went to a concert at Carowinds' Palladium in Charlotte NC many years ago. I think the headline act was Chicago.
At the time I had never heard of the warmup band: Bela Fleck and the Flecktones