I saw the Pogues headline at City Gardens, and the next night they opened for U2 at Madison Square Garden. They did a similar performance (different set list), but in MSG the lights were on in the stadium and maybe 20 people spread throughout the massive arena were paying attention and dancing.
I also saw Bob Dylan open for The Grateful Dead at Giants Stadium, and The Flaming Lips open for the Canadian hardcore band The Dayglow Abortions, also at City Gardens.