I was at a Chicago Symphony concert recently where the first piece, Samuel Barber's Adagio For Strings, completely upstaged the main event, Shostakovich's 5th Symphony.
'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.