Chesterfield's Caleb Folan grabbed a late winner to bundle West Ham out of the Carling Cup and turn up the heat on their under-fire boss Alan Pardew, as they beat the Premiership side 2-1.
The giant striker, who had been a handful all evening, slammed the ball home from close range in the 87th minute to condemn the faltering Hammers to an eighth straight defeat.
Marlon Harewood had given Pardew's men a scarcely-deserved fourth-minute lead but Chesterfield - 15th in League One - dominated thereafter.
They levelled in the 54th minute through Colin Larkin and Folan capped a sensational performance from Roy McFarland's side with the late winner.
After reaching the FA Cup final and qualifying for Europe last season, the Hammers have this term taken only five points from nine Premiership games and were dumped out of the UEFA Cup at the first hurdle.
This latest humiliation inflicted by McFarland's pugnacious side was no less than the below-par Hammers deserved.
Chesterfield were vibrant from the outset and came desperately close to opening the scoring in the third minute. With a combination of searing pace and clever footwork, Folan glided past two West Ham defenders and slipped in Larkin.
The striker beat the offside trap perfectly and hit a left-foot shot which drew a fine stop from Robert Green.
Barely 60 seconds later the Hammers forged ahead as Harewood ended his side's morale-sapping goal drought with his first strike of the season.
Reuben Hazell's weak header from a long West Ham clearance fell to Harewood on the edge of the 18-yard box and his right-foot volley fizzed into the net.
It was West Ham's first goal since Bobby Zamora's equaliser against Aston Villa on September 10 - and came distinctly against the run of play.
But the home side's response was impressive and after Zamora threatened twice in the space of a minute after breaking clear, McFarland's men engineered another opening.
The Spireites stretched West Ham down the left flank and when the ball eventually found its way to Paul Hall at the far post, his low cut-back failed to find a team-mate.
What Chesterfield lacked in technical ability they more than compensated for in sheer guts and determination.
One man in particular enjoyed the occasion - midfielder Derek Niven - and he emerged at every opportunity to collect possession and orchestrate his side's next move.
Having scored a classic long-range winner against Manchester City in the previous round, he came close to emulating the feat with another thunderous 25-yard strike which flew just wide of Robert Green's left-hand post.
And four minutes before the break he was at it again, this time hitting a low daisycutter which deflected off Anton Ferdinand before Green scrambled the ball to safety.
Chesterfield claimed a penalty in first-half injury-time when Mark Allott's left-foot shot struck Ferdinand's arm - but referee Lee Probert ignored the appeals.
Pardew replaced Harewood with Teddy Sheringham in an apparent bid to retain possession - but the Hammers boss was left stunned as Chesterfield equalised.
Folan was the provider, curling in a delightful left-foot strike from 18 yards which Green repelled with a fine fingertip save onto his post. However, Larkin reacted quickest, gobbling up the rebound left-footed.
The Hammers were restricted to playing almost exclusively on the counter-attack and one such move led to Sheringham forcing a corner and then seeing another effort blocked.
Pardew also brought on Matthew Etherington and Paul Konchesky but Chesterfield continued to probe and Kevan Hurst's powerful strike from distance forced Green to concede another corner.
With seven minutes remaining, Zamora was denied by Spireites goalkeeper Barry Roche.
And four minutes later Folan capitalised on a poor defensive clearance from a right-wing free-kick, sweeping the ball to spark wild celebrations at Saltergate.
