Over the past two to three seasons, Thibaut Courtois has been one of the very best goalkeepers in Europe.
The Belgium international, who has spent the last three campaigns on loan at Atletico Madrid from Chelsea, has been a model of consistency in La Liga.
However, the 21-year-old made possibly the worst mistake of his career on Tuesday, when he inexplicably allowed a header from his defender Toby Alderweireld to loop over him and into the back of the net.
Courtois didn't need to be told that he'd made a bad error. The youngster sank to his knees and put his head in his hands immediately after committing the blunder.
Atletico have already qualified for the Champions League knockout rounds, so Courtois' error was not too costly.
However, it did allow Zenit to earn a 1-1 draw, denying Atletico a fifth straight group stage win in the process.