Arsenal lost at home for the first time since May as a 2-0 defeat to West Ham cost Mikel Arteta's team the chance to return to the top of the Premier League.
Goals by Tomas Soucek in the 13th minute and former Arsenal player Konstantinos Mavropanos in the 55th - combined with a stout defensive display - gave West Ham a surprising win at Emirates Stadium and continued a midseason blip by the Gunners.
That's just one win in its last four league matches for Arsenal, which was in first place for Christmas and needs to beat Fulham away on Sunday to be in that position heading into 2024.
Arsenal is currently in second place and two points behind Liverpool, which doesn't play again until New Year's Day.
It could have been an even bigger win for West Ham but Said Benrahma had a penalty saved by Arsenal goalkeeper David Raya in the sixth minute of stoppage time.
There was some controversy over the first goal, with cameras not showing definitively if the ball was out of play when Jarrod Bowen turned it back into the area for Soucek to force home.
Bowen's leg blocked the best sight of whether any of the ball was in play and the referee stayed with the onfield decision of giving the goal.
A similar situation occurred with Arsenal at Newcastle, which scored the only goal of their game at St. James' Park on Nov. 4 amid debate over whether the ball was in play as a cross came in for Anthony Gordon's strike.
West Ham lost Lucas Paqueta to an apparent left hamstring injury just after the half-hour mark - the Brazilian had been a doubt at the start of the game - but losing one of their key players didn't put the visitors out of their stride.
Bukayo Saka hit the post for Arsenal but the home side lacked its usual energy in attack and conceded again when James Ward-Prowse's corner was flicked home off the underside of the crossbar by Mavropanos, whose celebration was muted because of his previous links to Arsenal.
Arsenal last lost at home in mid-May to Brighton, a defeat that virtually ensured the title last season would be heading to Manchester City and not the Gunners.
