Kamaru Usman
Usman was on course to eclipse Georges St-Pierre as the greatest UFC welterweight champion of all time. He fought the UK’s Leon Edwards in a title fight in Salt Lake City. Edwards had a good first round but Usman settled and dominated his rival. People began to leave early into the fifth round because the fight appeared to have run its course.
Then, out of nowhere, Edwards smashed Usman in the head with a brutal kick. It was perfect timing because he waited for Usman to drop to one side without keeping his guard up. This career-defining mistake changed Usman’s legacy and meant that he’d never be the greatest of all time (via MMA Fighting).