Bafetimbi Gomis scored a late goal against the run of play, and Swansea City surprised Arsenal 1-0 on Monday night.
Gomis' header appeared to be stopped by David Ospina and was only given after the assistance of goal-line technology.
Replays showed the ball clearly crossed the line before Ospina got his hands to it.
Arsenal controlled possession for nearly 70 percent of the game but could not find a breakthrough.
The Gunners missed a chance to go even with Manchester City in second place and remain three points back with a game in hand.
