People always talk about Mike Tyson's career, but I feel they do not give enough credit to guys like Buster Douglas and Evander Holyfield.
Douglas proved that Mike Tyson was actually human, and put a chink in his armor. Holyfield then stripped him completely naked. The mark for any fighter, since 99.9% never go undefeated, is how you handle your first loss. His loss to Douglas was the beginning of the end.