I once saw a mime open for a band. At the time, I figured the group did it so they wouldn't be upstaged. I guess it backfired because all these years later I remember the mime, but couldn't tell you the band.
Sometimes the opening band doesn't work simply because it's such a mismatch with the headliner. Acoustic folk singer Sharon Van Etten tells the story of playing an all-day metal festival. She's a wonderful artist, but tell that to the headbangers who booed her.
Sometimes the opening band is awful just because it's on a big stage way too soon, long before the group is ready. And sometimes the opening act sucks, because, well, it just does.
Who's the worst or most inappropriate opening act or band you ever saw? What made it so painful?