Charlie Austin shattered Arsenal's 22-game unbeaten run as the Southampton striker punished a howler from Gunners goalkeeper Bernd Leno to seal a shock 3-2 win on Sunday.
Austin settled a thrilling Premier League clash at St Mary's when he headed home with just five minutes left after Leno misjudged Shane Long's cross.
Southampton had twice taken the lead through Danny Ings, only for Henrikh Mkhitaryan's brace to draw Arsenal level on each occasion.
Leno's mistake and Austin's predatory finish handed Arsenal their first defeat in 15 league games and brought a stunning halt to their longest unbeaten run in all competitions since 2007.
Unai Emery's fifth placed team hadn't lost since a 3-2 defeat at Chelsea in August and the north Londoners' first setback since the summer leaves them two points behind the fourth placed Blues, who won at Brighton on Sunday.
While it was a bitter afternoon for Arsenal, Southampton's first win in 15 matches in all competitions was cause for much celebration.
Southampton manager Ralph Hasenhuttl, taking charge of his first home game, danced a jubilant jig on the pitch at full-time after securing the maiden victory of his reign.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 on Sunday.
The Belgian provided his ninth assist of the season for Pedro Rodriguez 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.
Brighton had only previously been beaten at home by Tottenham Hotspur this season.