Thomas Tuchel was left cursing the 'big mistakes' made by Chelsea as they slipped to a 3-2 defeat at West Ham.
The Blues boss counted the cost of two blunders by goalkeeper Edouard Mendy as they were beaten for only the second time in the Premier League this season.
Chelsea were leading through Thiago Silva's header when Mendy made an almighty mess of a weak Jorginho backpass and ended up bringing down Jarrod Bowen, allowing Manuel Lanzini to equalise from the spot.
Mason Mount's sublime volley put Chelsea back in front, but Bowen pegged them back again before Mendy pushed a speculative Arthur Masuaku effort inside his own near post to gift West Ham a notable victory and cast further doubt on the Blues' title credentials.