Tyrone Mings' mistake cost Aston Villa their first loss in the Champions League as they were defeated 1-0 by Club Brugge.
The defender accidentally picked up a short goal-kick by Emiliano Martinez without realizing that referee Tobias Stieler had restarted the game.
Brugge's captain, Hans Vanaken, converted the resulting penalty in the 52nd minute to secure the win for the Belgian champions and halt Villa's three-game winning streak in the Champions League.
It was a moment of confusion by Mings, who was making his debut in the competition. Comparisons were made with Arsenal's Gabriel, who was not penalized in a similar situation against Bayern Munich in the previous season.
The incident overshadowed Unai Emery's team's subpar performance, having now suffered three consecutive defeats in all competitions.
Despite the setback, Villa still have a good chance of qualifying directly for the knockout stage halfway through the group phase.
Their brightest opportunity came in the 21st minute but Watkins' shot missed the target. They struggled to create clear chances throughout the game.
Brugge seized control and had several scoring opportunities in a short period, with Mings making a crucial goal-line clearance before his costly error.
Vanaken's successful penalty kick proved to be the decisive moment as Villa failed to mount a significant comeback.
The Belgian side, led by former Haverfordwest manager Nicky Hayen, secured a historic victory over an English opponent.