Robin van Persie and William Gallas had put the Gunners on the way to what looked like being a certain victory, before Carlton Cole's diving header and an Alessandro Diamanti penalty salvaged an unlikely, late point for ten-man West Ham.
In-form Arsenal arrived at Upton Park having lost just two games in 14 league and cup outings and, following Tuesday evening's Champions League draw at AZ Alkmaar, Arsene Wenger had named an unchanged side.
Without a Premier League victory since they beat Wolves on the opening day of the season, wobbling West Ham kicked-off this dogged derby in 19th-place, still looking for their first home win of the campaign.
And following last week's disappointing defeat at Stoke City, gambling Gianfranco Zola made three changes as Guillermo Franco was handed his first start alongside Scott Parker and Jonathan Spector, who each came in for substitutes Diamanti, Radoslav Kovac and Julien Faubert.
Certainly, the Italian's shuffle of the pack looked to have revitalised the Hammers during an opening quarter-hour that saw Cole and Jack Collison threaten.
But with West Ham more than holding their own, van Persie then popped up on the edge of the six-yard box to produce a killer card of his own, after the red-faced Robert Green palmed the overlapping Bacary Sagna's right-wing cross onto the left boot of the unmarked Dutchman, who easily claimed his sixth goal of the season.
Ten minutes before the interval, more doom and gloom spread through the East End when van Persie floated over an inch-perfect corner which the in-rushing Gallas met with a bullet-header that almost decapitated Parker on the line.
The Frenchman's fourth goal of the season made Zola's half-time team-talk even harder and with West Ham still going nowhere after the break, the Italian introduced the orange-booted, shaven-headed Diamanti, who wasted no time letting fly with a speculative 35-yarder and a 20-yard curler.
But as the final quarter-hour approached, Arsenal were left to regret their failure to bag a show-stopping third as the Diamanti-inspired West Ham somehow turned the game on its head.
First, the sparkling Italian substitute whipped in a sizzling 25-yard free-kick that forced Vito Mannone into his first meaningful save of the afternoon, but the headlong Cole reacted first to nod the keeper's parry over the line from six yards to give Hammers just a glimmer of hope.
Having collected his fifth goal of the season, Cole then found himself bundled over by Alex Song and despite the consequently booked Mannone's attempts to put him off, Diamanti defiantly drilled the consequent penalty into the bottom right-hand corner to give West Ham an unlikely leveller.
That made for a frenetic finale in which Parker was harshly dismissed for fouls on Eboue and then van Persie but, despite introducing both Nicklas Bendtner and Eduardo for the final minutes, the gutted Gunners found themselves paying for those few minutes of late madness that saw them turn three certain points into a solitary one.