Arsenal missed the chance to go back to the top of the Premier League after losing 1-0 away at Aston Villa.
Unai Emery exacted revenge on his former side by clinching a club-record 15th successive league win - a run which started after losing 4-2 to the Gunners last season.
Villa's victory ensured they moved to within two points of first-placed Liverpool, who completed a dramatic comeback at Crystal Palace at lunchtime.
Bukayo Saka spurned an early chance to take the lead at one end and was duly punished when Villa went down the other end and broke the deadlock with five minutes played. Leon Bailey twisted and turned his way past a couple of Arsenal bodies before pulling the ball back for John McGinn to finish after keeping his cool from close range.
Arsenal were looking disjointed and were making basic errors across the pitch, though Martin Odegaard brought a fine save out of Emiliano Martinez and Gabriel Martinelli was causing Villa's right-hand side problems.
The Gunners' first-half struggles were summed up by William Saliba passing the ball out of play for a Villa corner under little pressure.
On the other side of half-time, Arsenal wanted a penalty when Gabriel Jesus' ankle was hooked upwards by compatriot Douglas Luiz, but even after a VAR check their appeals were dismissed.
Arsenal struggled to break Villa down, but as the game entered stoppage time, the Gunners thought they had a leveller. Kai Havertz converted at the back post from a deep cross, but he was judged to have handled the ball before it crossed the line.
Arsenal fans were up in arms (no, do not pardon the pun) about the decision as the ball also inadvertently struck defender Matty Cash's elbow, but the rules regarding handballs for goals are different for attackers.
After nearly ten minutes of added time, Villa's momentous win was secured and Arsenal were consigned to a devastating defeat.