Tyrone Mings' calamitous error condemned Aston Villa to their first Champions League defeat as they lost 1-0 at Club Brugge.
The centre-back inexplicably picked up a short goal-kick by Emiliano Martinez without knowing referee Tobias Stieler had restarted the game.
Brugge captain Hans Vanaken converted the resulting 52nd-minute penalty to earn victory for the Belgian champions and end Villa's three-game winning start to life in the Champions League.
It was a moment of madness by Mings, making his first start in the competition, but he may feel hard done by as Arsenal defender Gabriel was spared in the exact same circumstances against Bayern Munich in last season's quarter-final.
On that occasion Swedish referee Glenn Nyberg did not penalise, saying it was a "kid's mistake" and not in the spirit of the game to award a penalty.
Stieler and the VAR did not have the same outlook and Mings was punished.
The incident will divert attention from what was a below-par performance by Unai Emery's men, who have now lost their last three games in all competitions.
They were aiming to create Champions League history by becoming the first team to win their first four games in the competition without conceding a goal.
But they mustered just one shot on target with Ollie Watkins and Jhon Duran barely getting a sniff.
Despite the blip, they remain in strong position to earn direct qualification for the last 16 at the halfway stage of the new league phase.
Their best moment came in the 21st minute as they fashioned their best moment of the match.
John McGinn fed Watkins and the England international cut inside but dragged his shot just wide of the post.
McGinn then put a header over the top from Boubacar Kamara's cross as Emery's men tried to gain the ascendency.
But it was Brugge that became dominant and had four presentable chances in a 10-minute spell.
First Ferran Jutgla rattled the near post with a first-time shot, with Christos Tzolis forcing Martinez into a sprawling save.
Then Mings came to the rescue as he cleared Casper Nielsen's flicked header off the line with Tzolis firing straight at Martinez with the follow up.
Mings' night went downhill from there, though, as his incredible gaffe came in the 52nd minute.
Club Brugge are awarded a penalty after Tyrone Mings picks the ball up inside his own area 😳
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Martinez took a quick goal kick and Mings, who was not looking at the time, quickly picked the ball up and put it back down inside the six-yard box.
But German referee Tobias Stieler ruled play had restarted and pointed to the spot, with Vanaken sending his spot-kick down the middle.
The Belgian champions, coached by former Haverfordwest manager Nicky Hayen, pushed for a second and Skov Olsen fizzed an effort over.
Villa, who head to Liverpool in the Premier League at the weekend, huffed and puffed but Boubacar Kamara's shot off target was their best effort as Brugge held on for a first victory over an English club.
