Swansea climbed out of the relegation zone for the first time in three months as Petr Cech's dreadful clanger helped condemn Arsenal to a 3-1 defeat at the Liberty Stadium.
Cech's scuffed kick allowed Jordan Ayew to roll home a 61st-minute winner after Sam Clucas had put Swansea on level terms, instantly cancelling out Nacho Monreal's first-half opener.
Clucas claimed his second three minutes from time to finish off Arsenal, who hope to add to their squad before Wednesday night's transfer deadline with Pierre-Emerick Aubameyang set to swell their striking options.
But the Borussia Dortmund forward would not have been impressed with his new team-mates, who did not manage a goal attempt in the opening half-hour and were often second best against the Swans.
Olivier Giroud - who could be set to join Chelsea on deadline-day - and new boy Henrikh Mkhitaryan were sent on in the second half to try and salvage matters.
But Swansea are a different proposition under Carlos Carvalhal, who has taken 10 points from 15 since succeeding Paul Clement, and this victory steered them out of the bottom three for the first time since November 4.
Shkodran Mustafi's hesitation allowed Nathan Dyer to set up Ayew for a seventh-minute opportunity which was deflected wide.
Moments later Arsenal were grateful to Aaron Ramsey's brilliant intervention which prevented Alfie Mawson from converting Dyer's cross and adding to his match-winner against Liverpool last week.
After Ki Sung-yueng and Mawson went close for Swansea, Arsenal's first shot in anger arrived after 30 minutes when Alex Iwobi tested former Gunners goalkeeper Lukasz Fabianski.
Ayew's brilliant weaving run had Arsenal on the backfoot again, but the visitors were ahead after 33 minutes.
Mesut Ozil's delightful pass picked out Monreal's run at the far post and the Spaniard stabbed past Fabianski for his fourth goal of the season.
But Swansea were level inside 60 seconds as Mawson slipped in Clucas into the left-hand channel and, just like he had done at the Emirates in October, he beat Cech to end the goalkeeper's ambition of keeping a 200th Premier League clean sheet.
It was no more than Swansea deserved and they might have been ahead when Leroy Fer robbed Ramsey and advanced to the edge of the area.
Fer should have slipped in the unmarked Ayew, but he went for glory instead and Cech held his effort comfortably.
Arsenal were more assured at the start of the second half but were fortunate when Clucas failed to direct Kyle Naughton's cross goalwards.
Ozil was becoming prominent and he was involved in the same move three times before Fabianski grasped his low effort.
But Arsenal conceded in horrendous fashion when Mustafi dawdled before playing a back pass to Cech in the greasy conditions.
Cech's awful clearance went straight to Ayew, who made no mistake to roll home his fifth goal in nine games.
The goal unnerved Cech as he soon struggled to cope with a Ki free-kick, palming it away for a corner when there was no Swansea player near him.
Swansea should have extended their lead when Mustafi's careless header went straight to Dyer and he struck the outside of the post from 14 yards.
But Ayew scorched past the helpless Mustafi four minutes from time and Clucas reached the near post first to beat Cech again.