Leon Russell played at a small club down the street a few years ago. I was never really a big fan, but would like to have seen him. It was going to be a bit inconvenient to get there on time so I just decided I'd catch him the next time he was in town. A year later he was gone. That happened with another musician I wanted to see (Jesse Winchester) and it was then I decided I was not going to let any more opportunities pass by - even if it meant I was just going to get there in time to hopefully see an encore.
I was in Boston when M,D.& E played the Tea Party-- but the marquee just read "Joe Cocker" so I thought it was the Grease Band and didn't make the effort. I also missed the last time Hendrix was there, but that was okay-- I went to see Delaney and Bonnie at Harvard Stadium instead, without Clapton or Leon or Mason or Coolidge; but with Duane Allman and Gram Parsons instead, and those were the only times I saw them, so...