Here's two more....

Van Halen for Black Sabbath in late 70's. This was just after their first album came out and all I knew about them was their version of "You Really Got Me". Ozzy and company was good, but VH just blew them away.

I went to a Blues show featuring John Lee Hooker back in the late 80's and one of the opening acts was Robert Cray. He was awesome and shortly thereafter "Smoking Gun" started to get a lot of airplay.

Here's one that doesn't count: I went to see Be Bop Deluxe in '78 or so when they came to the states as the opening act for a Blue Oyster Cult tour. Got my tickets, took a bus home from college, got settled in my seat and waited....and waited.....waited...until they announced that there had been an accident on the NYS Thruway involving Be Bop Deluxe's equipment truck, and that they were unable to play that night. Arrrrgh! But BOC played an extra long set and they were pretty good (small consolation!)