Blackburn Rovers came from behind to beat Arsenal 2-1 at Ewood Park on Monday and leave third place in the English Premier League up for grabs.
David Dunn cancelled out Robin van Persie's opener before captain Christopher Samba scored the winner to stun the visitors.
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A win for Arsenal would have confirmed third place in the Premier League and direct qualification for the Champions League group stage.
But north London rivals Tottenham can now move to two points behind them if they win at Manchester City on Wednesday, meaning Spurs could pinch third place should they win and Arsenal lose in Sunday's final round of matches.
The win saw Blackburn overtake Fulham, Sunderland and Stoke and move into 10th place in the table.
Netherlands striker van Persie put Arsenal ahead in the 13th minute with his first goal since October 31 following his five-month ankle injury lay-off.
But midfielder Dunn levelled in the 44th minute with a tap-in at the far post as the Gunners failed to deal with a Rovers corner, and Congolese centre-back Samba scored the winner with a 68th-minute header.
Arsenal manager Arsene Wenger said his goalkeeper Lucasz Fabianski had been impeded both times.
"Their main purpose every time is to stop the goalkeeper, not go for the ball, and both times for both goals that happened," he said.
"I think he had no chance as a goalkeeper, he was pushed every time."
Rovers manager Sam Allardyce admitted targeting the Poland stopper.
"We know their weakness is balls into the box and certainly the goalkeeper of late, so we did that positively and in the end it has paid off for us with two goals and we've managed to win the game and finish the season at home with a fantastic victory against Arsenal," he said.
Samba added: "It's very difficult for teams to come and win here, it's been a little bit of a fortress at home and we wanted to have the result against one of the big four."
Carlos Vela wasted an early chance for the visitors, sliding it wide before Fabianski made a good save from a Morten Gamst Pedersen free kick.
Van Persie took an Arsenal corner which was cleared for another corner, this time taken by Samir Nasri.
His set-piece was flicked on by Bacary Sagna at the near post, with van Persie heading the ball into an empty net at the back post.
It was the Dutchman's eighth goal in eight games against Blackburn - more than he has notched against any other Premier League side.
Rovers goalkeeper Paul Robinson kept out a van Persie free kick on the half-hour mark.
And it was from another corner that the hosts equalised.
Pedersen's left-wing set-piece dropped to Ryan Nelsen. The New Zealand defender played a low ball to Keith Andrews, whose cross evaded Armand Traore, allowing Dunn to score with a simple finish at the far post.
Pedersen was at the heart of Blackburn's best attacks and he was downed in the penalty box by Sol Campbell but referee Martin Atkinson gave only a corner.
Wenger sent on Andrey Arshavin for the ineffective Vela in a bid to force a winner but within a minute Rovers were ahead.
Pedersen fired a corner into the box and Fabianski, surrounded by Blackburn players, failed to get to the ball.
The Rovers tactic worked as Samba was able to head in from three yards out.
Both sides then went on the attack but the hosts held out to move themselves into the top half of the table and give the Gunners extra reasons to hope Spurs do not win Wednesday's clash with Manchester City.
