Aston Villa blew the title race wide open with a surprise 1-0 victory over Chelsea at Villa Park on Saturday.
Fabian Delph scored his audacious winner on 82 minutes, but the headlines will be about a whole lot more after two Chelsea players were sent off along with manager Jose Mourinho.
Willian was the first, receiving his marching orders in the 68th minute, while Ramires was dismissed late on for a horrible challenge on Karim El Ahmadi.
In the melee that followed, Mourinho was also ordered to the stands by referee Chris Foy.
Chelsea had been in control for long spells of the game until Willian's dismissal, but unlike Manchester City earlier in the day—who saw Vincent Kompany sent off before going on to defeat Hull City 2-0—the Blues failed to overcome the setback.
It means their strong position at the top of the Premier League has been severely weakened, with Arsenal, Liverpool and City no doubt taking much encouragement from the game.
So what did we learn from Chelsea's Villa Park defeat? Bleacher Report highlights some key areas.