Tunesmith: I've noticed the same thing, and it irritates me, too. But it doesn't happen often.

I think the one that annoyed me was when The Battlefield Band did Bad Moon Rising. I think that was about 15-20 years ago. It wasn't egregiously awful, but it wasn't what I came to hear, either. And it was especially annoying that they put it at the end of their concert and that it got the biggest applause of the evening. It made me suspect that most of the audience were mainly rock/pop fans who were only pretending to like folk music, or who were experimenting with a genre that was outside their main line of interest--which might not be far from the truth. It's as if the band were saying, "Thank you for your patience in sitting through all this boring stuff. Now we'll give you something you'll really enjoy."

You know what similar thing also irritates me? Public television stations, in the US, when they have their semi-annual fund-raising drive, will put on programs that are totally different from what they show the rest of the time. But I suppose that's a topic for a different thread.