6. UFC 82: Arlovski vs. O'Brien - Arlovski's Final UFC Fight
Jake O'Brien was an undefeated 10-0 heavyweight entering into his fight with Andrei Arlovski at UFC 82, having won three in a row in the UFC, including a win over Heath Herring. Enter the Pit Bull, exit the undefeated record. This fight put Arlovski in an enviable position, as a premiere heavyweight on a winning streak coming off the final fight on his UFC contract. He entered the free agent market with a 10-4 UFC record overall, and signed with the fledgling Affliction MMA organization. O'Brien, for his part, would eventually drop to light heavyweight, only to run into another juggernaut of the sport: the up and coming Jon Jones.