James Ward-Prowse set up two goals on his West Ham debut and Lucas Paqueta scored a late penalty as the 10-man Hammers somehow secured a 3-1 win over Chelsea.
Ward-Prowse, the £30million signing from Southampton, created goals for Nayef Aguerd and Michail Antonio, either side Carney Chukwuemeka's equaliser.
But Mauricio Pochettino's expensively-assembled side will wonder how on earth they came away from the London Stadium empty-handed.
Enzo Fernandez missed a penalty and West Ham had to play the final 25 minutes a player down after Aguerd saw red.
Yet not only did David Moyes' side cling on for a first win of the season, they wrapped it up in stoppage time with Paqueta's spot-kick.
Ward-Prowse is renowned for his ability from set-pieces but surely not even Moyes could have expected such an immediate impact.
Seven minutes in and from his second corner in a claret and blue shirt, Ward-Prowse swung in a cross which Aguerd nodded in unmarked at the far post.
Chelsea have now conceded a goal in each of their last 13 matches, their worst run since 1996.
But they were making inroads at the other end with Nicolas Jackson eager to get in behind West Ham's back line.
One such run and cut-back was hacked away but when Ben Chilwell drove back in to the area, his cross was cleared by Kurt Zouma only as far as Chukwuemeka.
The 19-year-old steadied himself before lashing the loose ball past Alphonse Areola for his first Chelsea goal.
Paqueta was in the West Ham line-up despite being reportedly investigated by the Football Association for potential betting rule breaches.
The Brazil midfielder is said to be shocked by the probe, which is believed to centre around bets placed in his homeland on yellow cards awarded against him.
Paqueta did pick up a first-half booking for dissent after complaining about the award of a corner, but moments later he was inches away from scoring, dragging the ball down in a crowded penalty area only to clip a post.
Chelsea were gifted the chance to go ahead before half-time after Tomas Soucek tripped Raheem Sterling in the area, but Fernandez's penalty was saved by Areola.
Instead Antonio fired West Ham back into the lead eight minutes into the second half when he raced on to Ward-Prowse's cute ball over the top.
Pochettino turned to Chelsea's latest record signing, Moises Caicedo, for his debut on the hour mark, the Ecuadorian immediately sending a long-range shot wide.
West Ham were then reduced to 10 in the 68th minute after Aguerd was shown a second yellow card for needlessly fouling Jackson.
But Chelsea, it seemed, had run out of ideas and could only muster a deflected Noni Madueke shot which was tipped wide by Areola.
And the away fans left en masse even before Paqueta rubber-stamped the victory from the spot after Caicedo fouled Emerson Palmieri.
