OK, I have an opposite experience in the same vein. I went to see The Cult open for Metallica in 1990 and I really was not a fan of Metallica at all. I just wanted to see the Cult, and Ian Ashbury could not sing a lick at the show, which made it suck (as another aside, I saw them 4 times from 86 - 90 and twice they were phenomenal, and twice Ashbury had no voice at all). We decided to stay for the first song or 2 of Metallica and ended up staying the whole show and I bought all of their albums them. They absolutely rocked, one of the top 5 shows I have ever seen.
'they blew'em off the stage'
what concerts have you attended, where the warm up act blew the headliners away.....my examples...brownville station upstaging zz top to the point where people where chanting 'brownsville station!' during the littl ol' band from texas' set.....another..t.rex upstaging lynyrd skynyrd(yes there really was such a show), and the grass roots upstaging cream.