I saw the sad remnants of Big Brother open for a young Blue Oyster Cult. It was different.
I saw Def Leppard, as openers, mop the floor with Billy Squier.
The best was seeing Kansas fill in for an opener, Queen (who were supposed to open for Mott The Hoople), unannounced when Brian May got hepatitis a couple days before the Chicago show. I missed the very first song, but they were NOT well received. By the third song, the audience had been won over. Kansas was really great, right down to one of the guys announcing, "Hey, I didn't know I was supposed to be here, either! If I knew you wanted Queen, I'd have worn glittered underwear."