The Manchester derby is without question one of the fiercest in football.
Arguably the feistiest encounter between the two teams in the modern era came back in December of 2012.
The two Manchester clubs were battling for supremacy in the English top-flight at the time and they played out a hugely entertaining game at the Etihad.
United emerged from it with a season-changing 3-2 victory at the home of their arch nemesis, the win coming courtesy of a dramatic late free-kick from Robin van Persie.
Sadly, that goal resulted in things turning very, very nasty inside City's stadium.
Rio Ferdinand passionately celebrated Van Persie's winner in front of the home fans and as he was doing so, the Englishman was struck in the face with a coin.
And shortly after that incident, the United legend was targeted by a pitch invader as the situation threatened to descend into complete chaos.
Thankfully, Joe Hart was on hand to protect Ferdinand from the angry supporter and prevent what would have been utter mayhem at the Etihad.
Take a look at footage of the incident here...
Video: Hart protects Ferdinand from fan
It's hard to believe that actually happened, isn't it?
Ferdinand took a rather lighthearted approach to the matter in a Twitter post after the match.
He wrote: "Respect to Joe Hart for protecting that idiot who ran on!! Love the banter between fans but there is a point when it goes too far...calm it!"
Fair play, Rio.
In early 2013, the fan - identified as Matthew Stott - was given a three-year stadium ban and sentenced to 56 days in prison suspended for a year, per BBC.
As for United and Ferdinand, they went on to win the lift the Premier League trophy a few months later, pipping City to the accolade by a whopping 11 points in Sir Alex Ferguson's final season in charge.