Chelsea won the second leg of that series, advancing to the 2005 Champions League quarterfinals, but Frisk had had enough. Six months before that Chelsea match, he had been hit in the head with an object and left bleeding while refereeing a Champions League match between Roma and Dynamo Kiev. That match had been halted. And now, so had Frisk’s career.

Two weeks after the Chelsea episode, Frisk retired from refereeing at 42 to become a fulltime insurance agent, saying he felt like a “hunted animal” and could not put up with death threats from Chelsea supporters.

“I have been subjected to things that I couldn’t even imagine,” Frisk said at the time. “I love to referee and I have done it since 1978, but what has happened to me means it is not worth continuing. I won’t ever go out on a football pitch again. I am too scared. It is not worth it.”

Frisk declined a request for an interview. But he was not the only referee with such concerns. On May 6, 2009, Tom Henning Ovrebo of Norway presided over the second leg of a Chelsea-Barcelona Champions League semifinal at Stamford Bridge. Three minutes into added time, midfielder Andres Iniesta put a brilliant shot into the upper left corner of the net. With a 1-1 draw, and Iniesta’s tiebreaking away goal, Barcelona advanced to the final, which it would win over Manchester United.

Incensed, Drogba launched a vulgar tirade at the referee, caught on television, calling Ovrebo a disgrace. Bosingwa compared the referee to a thief. Drogba later received a four-match suspension for his outburst; Bosingwa received a three-game ban. Hiddink accused Ovrebo of missing three or four penalty kicks that should have been awarded to his team, adding that it was “worst refereeing performance I’ve ever seen.”