Yeah, I once(or twice) told the tale of a buddy asking me if I wanted to go see BLACK SABBATH 'cause he had an extra ticket due to his girlfriend getting sick and unable to go. I didn't( and never did) care for Sabbath and turned him down. I found out a couple days later that YES opened for them!
Had I known that ahead of time, I would have gone! But, I'd say those two bands weren't really on the same page.