Arthur Masuaku’s bizarre late strike sent Chelsea spinning to a 3-2 defeat at West Ham in a rip-roaring London derby.
Thomas Tuchel’s pace setters led twice through a Thiago Silva header and a sublime goal from Mason Mount, but they were pegged back both times by a penalty from Manuel Lanzini and Jarrod Bowen’s strike.
And with four minutes remaining substitute Masuaku, lurking out on the left wing, swung a speculative effort towards the Chelsea goal which Blues keeper Edouard Mendy pushed inside his own near post.
It was a second costly error by Mendy, who had also given away the penalty after he failed to deal with a backpass.
Not that West Ham were complaining, having got back to winning ways after three games without a victory to ensure they will stay in the top four this weekend.
There was little sign of the drama to come when Chelsea made the breakthrough in the 28th minute, with the type of goal West Ham have become synonymous with this season.
Mount swung over a corner and Silva’s thumping downward header beat Lukasz Fabianski, who got a hand to the ball but could only touch it onto the inside of his far post.
Moments later veteran centre-half Silva got back to his day job, instinctively being in the right place at the right time to block Vladimir Coufal’s shot on the line.
But West Ham were gifted a route into the match when Mendy made an almighty mess of collecting a weak Jorginho backpass and, having been challenged by Bowen, brought the Hammers forward down.
West Ham have had their issues from the penalty spot this season but David Moyes resisted sending on Mark Noble – who missed after climbing off the bench against Manchester United – for this one, and Lanzini confidently dispatched it into the top left corner, sending Mendy the wrong way.
However, a minute before the break Chelsea went back in front after West Ham gave the ball away cheaply in midfield.
Mount had acres of space to gather Hakim Ziyech’s cross-field pass, but still opted to hit it first time for a spectacular fifth goal of the season.
Left-back Ben Johnson seemed to be carrying an injury as he was unable to track Mount’s run, and he was immediately substituted for Masuaku.
Kai Havertz did not come out for the second half, having come off worse in a collision with ex-Chelsea defender Kurt Zouma, with Romelu Lukaku – who has nine career goals against the Hammers to his name – sent on.
But it was West Ham who struck next in the 55th minute with a fine finish from Bowen, across an unsighted Mendy and into the far corner to equalise again.
An injury to Zouma forced a reshuffle for Moyes, who had started with a back five to match up with Chelsea’s, only to run out of defenders.
They reverted to four at the back with midfielder Pablo Fornals joining the action, and Bowen was agonisingly close to putting West Ham ahead when he could not quite get enough on Michail Antonio’s ball across goal.
Instead it was the unlikely figure of Masuaku who collected Antonio’s header, floated it goalwards and duly inflicted Chelsea’s second defeat of the season.
