Arsenal overturned a two-goal deficit but could only earn a point as their Premier League title aspirations were dented at struggling West Brom.
Steven Reid put the hosts in front with a header from Chris Brunt’s corner.
Robin van Persie nodded against the bar before a horrendous mix-up between Manuel Almunia and Sebastien Squillaci gave Peter Odemwingie a simple finish.
Andrey Arshavin pulled one back with a superb strike before Robin van Persie forced the ball home to equalise.
The Gunners were encamped in West Brom territory late on but could not complete the comeback and were ultimately left to rue Almunia’s awful mistake.
The Spaniard was making his first league start since September in the absence of Wojciech Szczesny and Lukasz Fabianski, but there was no excuse for this howler.
Squillaci looked set to clear Youssouf Mulumbu’s long ball forward only for Almunia to come needlessly racing off his line, seemingly confuse his team-mate and allow the Nigerian to double his side’s lead.
This latest defensive calamity will raise further questions about boss Arsene Wenger’s ongoing reluctance to sign a top class goalkeeper.
Arsenal managed to launch a stirring riposte, but the result leaves them five points behind leaders Manchester United after their victory over Bolton – although the Gunners do have a game in hand.
Wenger’s men have now won only one of their last six matches in all competitions and it seems they are still struggling to recover from their recent Carling Cup final, Champions League and FA Cup defeats.
Arsenal did arrive at The Hawthorns unbeaten in 11 Premier League games but in West Brom they faced a side battling to avoid an immediate return to the Championship and resurgent under new boss Roy Hodgson.
The Baggies outclassed Arsenal in a shock 3-2 win at the Emirates Stadium earlier this season – Almunia’s last start – and they got at a nervous-looking Gunners rearguard from the outset.
Indeed, there was a sense of inevitability about West Brom’s opener as Reid easily evaded Aaron Ramsey to meet Brunt’s set-piece with a powerful header.
Arsenal only had themselves to blame and despite working hard to restore parity, their play was characterised by a general lack of urgency, creativity and precision in the final third.
Holding midfielder Denilson was particularly wasteful in possession but others were culpable too, and it was not until the 25th minute that the visitors created a chance of any note.
Gael Clichy crossed from the left for Van Persie to head against the crossbar and Baggies goalkeeper Scott Carson spread himself well to repel Ramsey’s close-range follow-up.
Arsenal dominated possession thereafter but rarely looked like breaching the home defence and it was hardly surprising to see Denilson replaced by Marouane Chamakh as Wenger switched from 4-5-1 to 4-4-2.
The move gave the Gunners greater presence up front but, again, they rarely troubled the West Brom defence and the momentum was soon back with the hosts.
The Baggies deserved to extend their advantage when Brunt poked Marek Cech’s low centre just wide after a stunning one-touch move, but a second goal was soon forthcoming.
Wenger had just introduced Nicklas Bendtner for Ramsey – replacing another midfielder with a striker – when Clichy lost possession to Mulumbu and Almunia did the rest.
The Gunners looked shell-shocked but eventually regrouped and, out of nowhere, Arshavin exchanged passes with Chamakh before drilling a magnificent first-time finish past Carson.
That gave West Brom an unwanted record of 28 Premier League games without a clean sheet – and their hopes of a first double over Arsenal since 1961-62 suffered a further setback when the visitors levelled.
Arshavin’s deep cross from the left hit the far post and fell to Abdoulaye Meite, who could not clear his lines and a Van Persie tackle saw the ball trickle over the line.
Arsenal then went for the jugular and Carson got down well to save Clichy’s low shot, while at the other end substitute Marc-Antoine Fortune saw a strike blocked by Squillaci’s last-ditch challenge.
But both sides were forced to settle for a point, which helps West Brom edge away from the bottom three.