There's been lots of great comebacks in MMA over the last year or so such as silva against sonnen, Russow against Duffee and Kongo against Barry. My favourite is Boetsch vs. Okami. Boetsch was struggling against Okami badly in the first two rounds. He then just went for it in the third round and ended up knocking Okami out with a minute to go. It's always easy to root for the underdog but I think most people expected him to get beat...which is what was happening until the third round.