Chelsea's defeat at Manchester City earlier in the day had offered Arsenal the incentive to close the gap at the top of the Premier League, ahead of their short trip to Stamford Bridge next Sunday.
Related ArticlesArsenal to play Newcastle in Carling CupWenger hails own record for nurturing talentNo change in transfer policy at ArsenalArsenal announce record profitsManchester City 1 Chelsea 0Man City v Chelsea: as it happenedBut a strangely inept performance left Arsenal and their fans stunned in the Emirates sunshine.
Two goals in two second-half minutes, from Peter Odemwingie and Gonzalo Jara, put West Bromwich Albion on course for their first win at Arsenal for nearly 27 years. And a third goal, from Jerome Thomas, handed the home side a potentially damaging reverse.
Samir Nasri replied with two late goals but by then the damage had been done.
Not since Manchester United won 3-1 in January have Arsenal tasted defeat but the score could have been worse had Manuel Almunia not saved a late first-half penalty.
After conceding the kick, Almunia saved from Chris Brunt. But he could do nothing to prevent Odemwingie from giving Albion a 50th-minute lead.
And the goalkeeper was at fault for the second goal, two minutes later, allowing Jara's ball to squeeze through his grasp.
Though Cesc Fabregas was absent with a hamstring injury, Arsene Wenger resisted the temptation to give Jack Wilshere another run-out following his impressive display in the 4-1 Carling Cup win over Spurs.
Instead, the teenager began on the bench ahead of Tuesday's Champions League trip to Partizan Belgrade.
Albion, who have started life back in the Premier League in reasonable fashion, were first to threaten.
Former Arsenal winger Thomas was allowed to run into the Arsenal area but sent a cross to nobody in particular.
And then, from the opposite flank, Brunt's delivery was almost met by Odemwingie.
Arsenal responded when Gael Clichy burst through from a deep position but saw his attempt blocked.
Albion survived again, in the 14th minute, when Emmanuel Eboue's cross was turned against an Albion post by defender Pablo Ibanez, with Andrei Arshavin finding the same left upright from the rebound.
Marouane Chamakh was next to threaten for Arsenal with a powerful header just off target.
Nasri went close again but Albion ensured it was not all one-way traffic when Nicky Shorey fed Odemwingie wide on the right and the Nigerian international's shot was deflected onto the outside of the right post.
And Arsenal survived before the interval when Almunia conceded a penalty, taking the legs of Odemwingie. But the Spanish goalkeeper, who was booked for the foul, responded with a fine save, diving to his right to deflect Brunt's kick to safety.
It was Almunia's fifth penalty save in six in the Premier League at the Emirates, but he was soon to be at fault.
Albion maintained their momentum with a deserved goal just five minutes into the second half.
Thomas beat Bacary Sagna on the left before feeding Odemwingie, who turned the ball over the line unchallenged.
If that was unexpected, what followed was remarkable, with Albion extending their advantage two minutes later.
Jara exchanged passes with Brunt, racing onto a back heeled pass. Spotting Almunia off his line, the Chilean slotted the ball between the goalkeeper and the right post, but the Spaniard was horribly at fault for the goal.
Arsenal made a double change soon afterwards with Wilshere and Tomas Rosicky replacing Eboue and Abou Diaby. And soon afterwards, Arsene Wenger gambled by switching defender Laurent Koscielny with striker Carlos Vela.
But another slip by Almunia allowed Albion to extend their lead in the 73rd minute.
The goalkeeper hesitated, allowing Brunt to cross for the outstanding Thomas to convert.
Nasri struck the cross-bar before clawing one goal back, in the 75th minute, and added a second in the first minute of added time, but Albion held on for a memorable victory.