MMA has come from the days of no rules fighting, but sometime there are idiots who disregard those rules.
At FIGHT UK: Last Man Standing, Ahmad Aswad and Nathias Frederick were competing, and out of nowhere Frederick lands and big shot that floors Aswad. Following up with ground and pound, the ref steps in to stop the fight, but Frederick continues attacking. Referee Leon Roberts takes the back of Frederick and slaps on the rear-naked choke.
Check out the end of the fight and the choke.
Video courtesy of BlecherReport