Manchester City welcomed Dortmund to the Etihad Stadium on Dortmund for their Champions League quarter-final first leg tie on Tuesday evening.
Pep Guardiola's side went into the game as the overwhelming favourites.
And they took the lead on the evening after just 19 minutes.
Emre Can lost the ball in midfield and Man City broke. Phil Foden's cross found Riyad Mahrez, who cut the ball back for Kevin De Bruyne.
Dortmund came battling back though and thought they had scored a vital away goal from Jude Bellingham.
The 17-year-old got to the ball ahead of Ederson before converting to an empty net.
However, the referee ruled out the goal as he deemed his challenge to have been a foul.
Replays showed that decision was very harsh. Bellingham clearly got the ball and Ederson then booted Bellingham.
VAR could not intervene as the whistle has blown before Bellingham's effort had crossed the line.
Watch the moment below:
That is just so, so harsh on Dortmund.
To make matters even worse for Bellingham, he was actually booked for that challenge.
Many raged on Twtter, including former Liverpool striker Stan Collymore.
"Embarrassing that this isn't a goal by Bellingham. Just embarrassing. Struggling to understand so many decisions in the sport today. This is just not fair or right or even remotely an infringement," Collymore wrote.
While Jadon Sancho, who is injured and is not playing for Dortmund, wrote on Twitter: "This Ref needs checking!"
That goal should never have been disallowed. It could be a massive decision in the outcome of the tie.