AC/DC with some Japanese metal band opening in Roanoke VA in the mid 80s (85 or so). They sounded horrible; the songs were practically unrecognizeable. Not sure if it was the band or the arena's acoustics (or both).
The Cars in Richmond, VA on their "Panorama" tour (81'ish) were disappointing; they stood there motionless, except for strumming their guitars, and sang word for word from the albums. No interaction with the audience at all.
And if opening acts count, definitely the Hives from Oct '07 was the worst ever. The most white-knuckled musical experience ever. There was no beat; yet the beat was everywhere. It was totally freaking my blood pressure out. That plus the FREAKY LIGHT SHOW with those lights that rise up and outward until you can't see your feet, like Close Encounters of the Third Kind, while you are balancing on the top row in the nosebleed section and trying not to tumble down the skinny stairs (free radio station tickets). I could not WAIT until they were gone. I know I'm "old", but my teen daughter didn't like them either.