Well, let's see... the most insulting was a concert that the Kingston Trio did about ten years ago. I realize that at their peak they were very limited, but like many people, I was introduced to a lot of folk music by them. I would never got to see them, but I was asked to provide an opening act for them at a big venue, and I recommended by friends Sally Rogers and Howie Bursen. Sally and Howie did a fine opening set, but when the Kingston Trio came out they might as well have just said "You're a bunch of pathetic loosers, trying to recapture your college drinking binge/make out days, and you'll eat up everything we sing, no matter how sloppy we are." And they were right. They got into long conversations about how drunk they'd gotten the previous night, and room service, and all sorts of idiotic stuff that had nothing to do with music, and then they'd launch into the most disjointed stab at Tom Dooley or Charlie and the MTA, and the crowd went nuts. After the concert, I was a little stunned at how awful they were, and everyone was walking out, grinning ear to ear, saying how great they were. It was also telling that I'd been running a folk concert series and festival for at least fifteen years in the same town, and yet I didn't recognize a single person in the audience who came to the concerts I ran. And I knew that even though there were 400-500 people there to hear the Kingston Trio, I'd never see one of them at the concert series that I ran... even if I booked Sally and Howie. It was like going to hear the Platters, when nobody in the group was in the original group. Pavlov's music. Ecccchhh!
Then, I saw Johnny Cash stinking drunk, where he could hardly stand up, with Dylan slumped on the stage with his back against the wall, trying to back him up. You could do a thread "Singers I've seen who were so drunk or drugged up that they could hardly stand, and the thread would run on from here to Cleveland." The spooky ones are those who sound God-awful when they are sober.