Tottenham Hotspur supporters who booed goalkeeper Guglielmo Vicario were told afterwards "they can't be true Tottenham fans" by boss Thomas Frank.
Spurs conceded twice in the opening six minutes to lose 2-1 to Fulham, their 10th Premier League home defeat of 2025.
But the main talking point was Fulham's second goal when Vicario lost possession well outside of his box.
He came out to deal with a long ball and took the ball towards the touchline.
However, instead of booting it out of play, the Italian turned and tried to clear, but slipped as the ball skimmed off Harry Wilson and was controlled by Josh King
King laid the ball off to Wales midfielder WIlson, who curled a shot into the goal from the touchline measured at 36.6 metres.
Moments later when the ball came to Vicario again, some Spurs fans booed him.
The team were booed off at half-time, with the club 2-0 down, and again at full-time.
One of those booing sessions really irritated Frank.
"I heard some of our fans apparently booed the incident and booed after, which, in my opinion is completely unacceptable," the Dane told BBC Sport of the fans' reaction to his goalkeeper.
"[They] can't be true Tottenham fans that do that. Fair enough booing after the game, no problem, but when we are playing, we are supporting each other, we are behind each other going forward."
Kenny Tete had given Fulham a fourth-minute lead before Wilson's strike - with Mohammed Kudus netting for Spurs in an improved second-half showing.
Former Premier League goalkeeper Joe Hart told Match of the Day the second goal was "completely avoidable".
"I do understand the fans' frustration," Hart added. " I know the role Vicario is playing. He's a great team player, he's a real leader in the dressing room but ultimately you are going to be judged by your actions.
"He was heavily involved in what turned out to be the winning goal."