West Ham United stunned Premier League leaders Chelsea with a 3-2 win on Saturday after coming from behind twice and then claiming all three points with a bizarre late goal from substitute defender Arthur Masuaku.
The Hammers clinched the win in the 87th minute when Masuaku caught out Edouard Mendy with what looked like a cross but the ball flew into the near corner for the Congolese full-back's first goal in his 96th Premier League appearance.
"It was very fortunate the goal, let's not kid ourselves. Truthfully I didn't think we played that well today, but we got the goals," West Ham manager David Moyes told BT Sport.
Chelsea had seemed to be in control of the game after 28 minutes when Thiago Silva scored with a header from a Mason Mount corner that West Ham goalkeeper Lukasz Fabianski could only touch on to a post before it went in.
Moments later, the 37 year-old Brazilian saved Chelsea at the other end when he cleared Vladimir Coufal's goal-bound shot.
But the European champions gifted the hosts a way back into the game in the 40th minute when Jorginho put his goalkeeper Edouard Mendy under pressure with a back pass.
Mendy brought down Jarrod Bowen for a penalty that was converted by Manuel Lanzini.
It took Chelsea only four minutes to get back in front when Mount, back to his best after an absence due to wisdom teeth problems, connected with a long pass from Hakim Ziyech for a perfect volley that caught out Fabianski at his near post.
Chelsea brought on Romelu Lukaku at the start of the second half to replace the hobbling Kai Havertz, giving the Belgium striker much-needed minutes in his quest for match fitness following an injury.
But West Ham drew level again in the 56th minute when Bowen fired in hard and low from the edge of the box.
As both sides pushed for a winner, Bowen could not convert a cross from Michail Antonio in the 75th minute while a Jorginho shot was deflected wide before Masuaku's surprise winner.
It was the first time Chelsea had conceded more than one goal in any competition this season.
"It is impossible to make so many mistakes and expect a result," Chelsea coach Thomas Tuchel said.
The defeat left Chelsea with a one-point lead at the top of the table over Manchester City, who visit Watford later on Saturday, and two points ahead of Liverpool who travel to Wolverhampton Wanderers.
West Ham are fourth with 27 points, six points behind Chelsea.