A late goal by substitute Charlie Austin gave relegation-threatened Southampton a thrilling 3-2 victory over Arsenal on Sunday in new Austrian manager Ralph Hasenhuettl's first home game.
It was the Londoners' first defeat in 23 games and meant they lost ground on fourth-placed Chelsea, who beat Brighton & Hove Albion away on Sunday, while Southampton moved out of the Premier League bottom three.
Danny Ings, back after injury, headed two fine goals in the first half from equally good crosses by Matt Targett and Nathan Redmond.
In between, Henrikh Mkhitaryan drew Arsenal level with a header of his own and early in the second half he scored via a big deflection.
Yet in the 85th minute Austin, who had come on for the tiring Ings, headed in a cross by Shane Long.
A first win in 13 league games since September 1 sent Southampton above Burnley into 17th place while Arsenal, who had last lost to Chelsea in the Premier League in August, remained fifth in the table.
Unai Emery's team had Laurent Koscielny starting in defence for the first time this season but they were uncertain at the back from the first to last minute against a side that at one stage this season went five league games without scoring.
Koscielny was at fault in failing to cut out Targett's cross from the left and losing Ings, who scored his first headed goal for three years.
The visitors briefly looked more composed and drew level when Nacho Monreal cut back a cross for Mkhitaryan to head in, and a minute later Pierre-Emerick Aubameyang was only denied by a late interception from Jan Bednarek.
Southampton went ahead again, however, before the interval. Redmond supplied the chance with a neat chip and once again Ings eluded his marker to score.
So Arsenal maintained their dismal record of never having been ahead at half time in the Premier League this season.
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As a consequence Emery made one of his regular half-time substitutions in sending on Alexandre Lacazette for wing back Hector Bellerin, and Aubameyang almost equalised within a minute.
Mkhitaryan's second goal was fortunate in the extreme, his shot taking a big deflection off Jannik Vestergaard.
Long, replacing Redmond, had a goal rightly disallowed for offside but then picked out Austin with a cross that goalkeeper Bernd Leno failed to deal with.
"It's great. The gaffer's brought in new ideas and it's not just changed things in the changing room but around the whole club," said match-winner Austin, adding "the next two games are massive and we've got to try to take six points."
Arsenal's Alex Iwobi said his side had not concentrated sufficiently and not defended well enough as a team.
"We're frustrated the unbeaten run is over," he said. "We had chances to win the game. But we should give ourselves some credit for going on the run."
Chelsea put a week of negative headlines for the behaviour of their fans behind them on the field thanks to an inspired Eden Hazard in a 2-1 victory at Brighton.
The Belgian provided his ninth assist of the season for Pedro to open the scoring and then chalked up his ninth goal of the campaign, but first in 13 games for club and country.
However, Chelsea failed to build on their 2-0 lead and endured a nervous finale after Solly March pulled one back with his first goal of the campaign.
Victory saw Chelsea open up a three-point lead over Arsenal in fourth and close back to within seven points of leaders Manchester City.
"We want to be top of the league at the end of the season," Hazard said. "We know Man City and Liverpool are good teams - we don’t want the gap to be big."