Well, the Super Bowl Half Time show from a few years ago that featured The Black Eyed Peas was pretty terrible, in my opinion. But if you're talking strictly about live performances we were actually present for, I've seen some pretty horrendous karaoke renditions, haha!
I don't think I've ever been to a live show/concert where I was disappointed... and I've been to a lot of live shows. Even most of the opening acts have been fairly decent.
That being said, if I HAD to choose the worst I've witnessed, I would have to say The Stepkids. They opened for Mates of State last year, and the act wasn't really that bad, I just wasn't familiar with their music, and it was a little out of this world for me. ;-)