Manchester United headed into Tuesday night's game against CSKA Moscow knowing top spot in their Champions League group was all but guaranteed.
Having won four out five games in Group A, the Red Devils only needed to avoid losing by four goals, which was never going to happen.
Jose Mourinho didn't take any risks, though, and still fielded a reasonably strong starting line-up that featured Romelu Lukaku and Paul Pogba.
But even with some of their big guns on the pitch, United were unable to break the deadlock and went 1-0 down in the 45th minute.
Mario Figueira Fernandes was played down the right-hand side and cut the ball back into the danger zone for Vitinho, whose strike at goal deflected off Alan Dzagoev and in.
But United's players weren't happy. Dzagoev appeared to be in an offside position when the ball hit his back and found the back of the net, as you can see in the video below.
You can see why United's players were angry and confronted the referee, with a lot of football fans equally as confused why the goal wasn't ruled out.
However, it turns out the referee made the correct decision. Here's why.
FIFA laws state that:
"Any defending player leaving the field of play for any reason without the referee's permission shall be considered to be on his own goal line or touch line for the purposes of offside until the next stoppage in play."
So, because Daley Blind was off the pitch when the ball hit Dzagoev without the referee's permission, he was technically playing the Russian onside.
The International Football Association Board (IFAB) also explain this rule, as you can see below.
ADVANCED OFFSIDE RULE EXPLAINED
Not that it mattered in the end, with Lukaku and Marcus Rashford scoring in the 64th and 66th minute respectively to put United in front.