Mikel Arteta was seven minutes from victory when Bernd Leno decided to leave his goal in pursuit of a high ball.
The goalkeeper’s intention was to punch clear the free-kick from Mason Mount and ease the pressure which had been building on his patched-up defence.
Leno punched at the air and the ball sailed on to Jorginho, who nursed it into an empty net on the half volley.
Arteta put his hands to his head. Like his players, he probably thought Jorginho should have been sent off for a second yellow card, moments earlier, when the Chelsea midfielder pulled at Alex Lacazette to halt a promising attack. Worse was to follow, however, as Arsenal reverted to their bad habits, refused to settle for a point, over-committed and were stung on the break, four minutes later.