Robin van Persie twisted free on the edge of the box and arrowed a fine shot past Joe Hart which hit the inside of the post—leaving Ashley Young free to tap home United's third.
His celebrations were short-lived, though, as the offside flag went up immediately to deny him the goal, and, in truth, neither he nor his teammates particularly appealed the decision—but the video above shows that they should have.
Pablo Zabaleta was clearly playing Young onside, and United should have been out of sight.
The goal remained disallowed, and United's problems were compounded just seconds later, as City went straight up the other end, and, despite the best efforts of David De Gea, Yaya Toure pulled a goal back for the home team.
United still lead 2-1, but a game which should have been out of sight and tied up, leaving United six points clear at the top of the Premier League, was suddenly back in the balance and needing a big last 20 minutes to claim victory.
There were several other incidents of note which the referee and his assistants affected during the course of the second half, but the offside decision and subsequent City goal have arguably been the biggest of them...so far.
Wayne Rooney scored both of United's goals in the first half.
The timeline of reaction on Twitter to the offside incident clearly showed how many viewers were in agreement with the decision initially—only for replays to soon prove it incorrect and incense the away support:
NO GOAL! The one time Ashley Young decides to stay on his feet and he's only offside. He won't do that again in a hurry— MirrorFootball (@MirrorFootball) December 9, 2012
Van Persie against the post and Young puts in the rebound - but the offside flag goes up.— Phil McNulty (@philmcnulty) December 9, 2012
Moments earlier, United had the ball in the net but Ashley Young's effort was ruled out for offside. Will the decision prove pivotal?— bet365 (@bet365) December 9, 2012
Young's goal was not offside. United denied a clear goal.— Manchester United (@MUFCanalysis) December 9, 2012
...first look at the Young no-goal showed him clearly offside. Replays from the other side showed him on thanks to Zabaleta.
— FOX Soccer Trax (@FOXSoccerTrax) December 9, 2012