Swansea City captain Ashley Williams's late goal earned his side a 2-1 victory at the Emirates Stadium on Wednesday that further dented Arsenal's Premier League title chances.
The central defender scrambled home 16 minutes from time as Arsene Wenger's side were made to count the cost of hitting the woodwork three times.
Joel Campbell had given the hosts an early lead, but Wayne Routledge levelled before the break and the south Wales side produced a determined second-half display to boost their hopes of avoiding the drop.
While they pulled six points clear of the relegation zone, Arsenal remain five points below leaders Leicester City.
Arsenal made three changes to the side that were beaten 3-2 at Manchester United on Sunday, with Per Mertesacker, Campbell and Olivier Giroud all returning.
Swansea were without head coach Francesco Guidolin, who was taken to hospital with a chest infection, so Alan Curtis took charge.
He made six changes to his side, with a Premier League debut for left-back Stephen Kingsley and a first start for Leroy Fer among the notable additions.
Arsenal had failed to score in four of their previous six matches at the Emirates Stadium, but quickly signalled their intent to ensure that sequence did not continue.
Francis Coquelin shot over the bar early on, while Giroud could not quite get a proper connection on Hector Bellerin's cross.
Alexis Sanchez was pulling the strings and he was unlucky not to open the scoring as he hit the post, seconds after an initial air shot, with former Arsenal goalkeeper Lukasz Fabianski left to watch and hope.
Wenger's side did break the deadlock 15 minutes in, with Campbell showing cleverness to strike the ball past Fabianski having seemingly lost his footing from Sanchez's pass.
It was the Costa Rican's fourth goal of the campaign, one of the other three having come in the 3-0 win over Swansea at the Liberty Stadium, and it immediately eased the tension around the Emirates.
Bellerin came close to a second from distance, with Fabianski just able to retrieve the ball before Aaron Ramsey could reach the rebound.
But Swansea were a threat on the break and former Queens Park Rangers midfielder Fer went close with a powerful drive from the edge of the area.
Campbell saw a deflected shot gathered by Fabianski, but Swansea went on to level with a goal out of the blue on 32 minutes.
Mesut Ozil lost out to Jordi Amat on the halfway line, with Wenger claiming a foul, but play continued and Jack Cork fed Routledge, who finished coolly past Petr Cech.
Arsenal's toils continued when Giroud volleyed against the bar after Mertesacker had headed on Ramsey's cross.
It was Swansea who looked brighter at the start of the second half as Andre Ayew shot over from the edge of the area, while they looked they much improved at the back.
Wenger replaced Campbell with Danny Welbeck and was met with chants of "You don't know what you're doing!" from his own fans, whose frustrations quickly increased.
Moments afterwards, Swansea squandered a chance to go ahead as Gylfi Sigurdsson rounded Cech from Ayew's pass, but dragged his shot wide.
At the other end, Arsenal hit the woodwork for a third time as Sanchez's deflected free kick had Fabianski beaten.
Swansea, though, went on to score a winning goal that could have huge ramifications for both ends of the table as Williams netted in scrappy fashion.
With Cech unable to claim the ball, he deflected in from Sigurdsson's free-kick in a moment that just about summed up Arsenal's day, ahead of Saturday's pivotal showdown with north London rivals