Fall Out Boy before hiatus
Blink-182 when they came back after hiatus
It has to be a band you had the chance to see but weren't able to for some reason.
I wish I'd seen FOB before hiatus. It's kind of funny, since they toured with Hey Monday and Metro Station (and All Time Low and Cobra Starship, I think) right before the hiatus. I really wanted to go to that tour, and I had the money and transportation available, but none of my friends were interested, so I missed it. After that, they went on hiatus. Now, if no one wants to go with me to a show, I'll go alone instead of missing out on an opportunity.
Also, once I friended someone on Last.fm and was looking at their concert list--it was huge and contained every concert in the past two or three years before that that I wanted to attend, but missed (usually, again, due to lack of interest from my friends).
In Warped 2010, Mayday Parade and Hey Monday were playing at the same time. I went with Mayday because I like them more, but looking back, I wish I'd seen Hey Monday because the crowd at Mayday was awful. The stage was just way too small for the crowd they gathered.