Arsenal's Premier League title bid was dented on Thursday night by a 2-0 home loss to London rivals West Ham United.
The Hammers had not won at the Emirates Stadium since August 2015 but goals from Tomas Soucek and former Gunners defender Konstantinos Mavropanos secured a rare victory.
It could have been even worse for Arsenal but David Raya denied West Ham a third goal by saving a late penalty kick from Said Benrahma.
Arsenal needed a win to go top of the table on Thursday but fell behind to a controversial goal in the 13th minute when Tomas Soucek fired home following a cutback from Jarrod Bowen.
The goal was then the subject of a VAR review that lasted three and a half minutes.
Multiple replays from various angles were studied in an attempt to confirm whether or not the ball had gone out of play before Bowen assisted Soucek.
But none of the available angles could offer definitive proof, so the officials stuck with the on-field decision and awarded the goal.
This was the second time this season that Arsenal had conceded a goal following an inconclusive VAR review.
The Gunners lost 1-0 at Newcastle in November when the VAR was unable to prove whether or not the ball had left the field of play moments before Anthony Gordon's winning goal.
West Ham's second goal on Thursday was far more straightforward. A whipped corner kick from James Ward-Prowse was headed powerfully into the net by Mavropanos, who had made eight first-team appearances for Arsenal between 2018 and 2019.
Arsenal dominated the game in terms of possession and chances, recording 30 shots to West Ham's six, with Alphonse Areola making eight saves.
But West Ham had the chance to make it 3-0 in added time after Declan Rice fouled Emerson inside the 18-yard box.
Benrahma stepped up looking to score his first goal of the season, only to be denied by a flying two-handed stop from Raya.
This was Arsenal's first home loss of the season. The Gunners had won seven and drawn two of their opening nine EPL matches at the Emirates Stadium.
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