Match ends, West Bromwich Albion 2, Arsenal 1.
Arsenal wasted the chance to go top of the Premier League as West Brom fought back from a goal down to beat them at The Hawthorns.
The visitors took the lead through Olivier Giroud, who headed in Mesut Ozil's free-kick from the right.
James Morrison volleyed past Petr Cech to level before Mikel Arteta diverted James McClean's cross into his own net.
Santi Cazorla could have secured a point from the penalty spot, but he slipped at the crucial moment.
Victory moves Tony Pulis' side up to 12th, while Arsenal's first league defeat since 19 September sees them slip to fourth.
Have Arsenal got the jitters again?
Some supporters of other clubs will have little sympathy for griping Gunners fans.
A club that consistently finishes in the top four, with a great assortment of attacking players, delivering attractive football and with a healthy transfer budget does not warrant a shoulder to cry on, surely?
But put yourself in the shoes of their fans. It is the hope that hurts.
The match at The Hawthorns was supposed to be the start of a winning run that would see them go on beat Norwich, Sunderland and Aston Villa. But, alas, they have stumbled once again.
The Gunners had 73% possession and 11 efforts on goal. Two of those should have been converted, with substitute Joel Campbell missing from four yards and Cazorla slipping as he went to take a spot-kick.
Then there is the growing injury list...
Gunners squad depleted again
Injuries have cost Arsenal time and time again in recent years.
Indeed, when Aaron Ramsey was sidelined in December 2013, the Gunners' title hopes were damaged.
They have a stronger squad now, but any team missing players of the calibre of Theo Walcott, Alex Oxlade-Chamberlain, Danny Welbeck and Jack Wilshere would find it hard to sustain a title challenge.
Here, midfielders Mikel Arteta and Francis Coquelin came off with injuries. Arteta had replaced Coquelin in the 14th minute.
Arsene Wenger's squad is not yet threadbare, but he cannot afford any more injuries over the next five weeks.
Pulis' men find their dogged touch
Yes, they rode luck and conceded the majority of possession to the Gunners, but the Baggies deserve full marks for resilience and belief.
Their previous two league games saw them leak five goals, but Jonas Olsson's introduction to the backline provided some much-needed muscle.
Bodies were flung in front of Arsenal players to deny them space, while Craig Dawson produced the block of the match when he blocked Kieran Gibbs from six yards out.
Man of the match - James McClean
What they said
West Brom manager Tony Pulis:
"There have been games here this year where we've been disappointed because we've lost. I thought Arsenal were very good, particularly second half when they ramped it up.
"Alexis Sanchez was just wonderful - his movement, his play. You've got Ozil and Cazorla who have wonderful talent. The lads deserve it for the effort and commitment, but we've played better and done better and not got the results."
Arsenal manager Arsene Wenger:
"It was a very bad afternoon for us. We had 70% possession, we scored an own goal, we missed a penalty, we gave away a goal from a set-piece, that is perfect to make the afternoon a nightmare.
"We missed open-goal chances. I believe West Brom did fight with 100% commitment as well and you have to give them credit for that.
"At 1-0 we had just to make sure they did not come back with an easy goal and that's where we were guilty, because apart from that I cannot fault the team who gave what they could until the last minute. We produced enough to win the game."
The stats you need to know
- Since joining Arsenal in 2012, Giroud has scored more headed goals than any other player in the Premier League (16)
- Ozil has provided an assist in seven consecutive Premier League appearances, the most in the division's history
- Chris Brunt has racked up 11 assists in the Premier League from set-pieces since the start of the last season, more than any other player
- Morrison has scored three Premier League goals against Arsenal - against no other side has he managed more (also three v Manchester United)
- Cazorla failed to score from the penalty spot for the first time in the Premier League, having converted each of his previous six spot-kicks
Arsenal face Dinamo Zagreb in the Champions League this week before are away to Norwich on Sunday. The buoyant Baggies are away at West Ham next Sunday.
- 13MyhillBooked at 90mins
- 11BruntBooked at 83mins
- 7MorrisonBooked at 43minsSubstituted forLambertat 77'minutes
- 29SessegnonSubstituted forGardnerat 61'minutes
- 33RondónSubstituted forBerahinoat 68'minutes
- 24BellerínBooked at 31mins
- 34CoquelinSubstituted forArtetaat 14'minutesSubstituted forFlaminiat 49'minutes
- 3GibbsSubstituted forCampbellat 63'minutes
- 12GiroudBooked at 42mins
- 54Reine Adelaide
- Mark Clattenburg
- Shots on Target
Second Half ends, West Bromwich Albion 2, Arsenal 1.
Attempt saved. Laurent Koscielny (Arsenal) left footed shot from the centre of the box is saved in the bottom right corner.
Alexis Sánchez (Arsenal) wins a free kick in the defensive half.
Foul by James McClean (West Bromwich Albion).
Boaz Myhill (West Bromwich Albion) is shown the yellow card.
Hand ball by Rickie Lambert (West Bromwich Albion).
Corner, Arsenal. Conceded by Chris Brunt.
Per Mertesacker (Arsenal) wins a free kick in the defensive half.
Foul by Saido Berahino (West Bromwich Albion).
Santiago Cazorla (Arsenal).
(West Bromwich Albion) wins a free kick in the defensive half.
Penalty missed! Bad penalty by Santiago Cazorla (Arsenal) right footed shot is too high. Santiago Cazorla should be disappointed.
Chris Brunt (West Bromwich Albion) is shown the yellow card for a bad foul.
Penalty Arsenal. Alexis Sánchez draws a foul in the penalty area.
Penalty conceded by Chris Brunt (West Bromwich Albion) after a foul in the penalty area.
Foul by Héctor Bellerín (Arsenal).
Saido Berahino (West Bromwich Albion) wins a free kick on the left wing.
Substitution, West Bromwich Albion. Rickie Lambert replaces James Morrison.
Offside, West Bromwich Albion. Jonas Olsson tries a through ball, but Saido Berahino is caught offside.