One of the biggest unspoken rules in combat sports is that you don't touch the ref. That rule was broken in a pretty massive way this past weekend during German promotion Aggrelin's 19th event (via MiddleEasy).
During the main event, Nihad Nasufovic caught Wilhelm Ott in a rear naked choke that had Ott tapping. But instead of stopping the fight, the ref simply watched as Ott tapped and didn't step in to stop things.
In the end it was Nasufovic who released the hold. And then things got crazy. Ott, obviously pissed over the non-stoppage, gave the referee a huge shove and then a big overhand right.
To his credit, the ref simply covered up and didn't fight back as the cage filled with members of both team. Somehow the entire scene didn't devolve into a brawl, because as you know sometimes these things happen in MMA.