Jon Jones is the greatest light heavyweight in the history of MMA and the pound-for-pound best fighter on the planet. He would be undefeated right now if not for a disqualification loss against Matt Hamill in 2009.
In the first round of that fight, Jones got mount on Hamill and rained down blow after blow. Hamill would not give up, though, so Jones resorted to dropping elbows straight downward onto his his skull. Those elbows -- dubbed "12-to-6" elbows in the Association of Boxing Commissions (ABC) bylaws -- are illegal.
With little warning, referee Steve Mazzagatti stepped in and stopped the fight.