Pep Guardiola celebrated taking charge of Manchester City for the 100th time in the Champions League on Tuesday, but it was Bayer Leverkusen and their fans that were left dancing at full-time.
The Bundesliga side would have been expecting a tough evening upon their delayed arrival in England, but it turned into a memorable 2-0 victory to leave Guardiola stunned on his milestone moment.
The Spaniard said in Monday's pre-match news conference that there had been "more disappointments than good moments" for himself and City in Europe's elite club competition and this was another of those lows.
The defeat was self-inflicted - Guardiola gambled with his starting line-up by making 10 changes to the side that lost at Newcastle on Saturday and the decision backfired in spectacular fashion.
"I have to accept it," Guardiola told TNT Sport of any criticism to the number of changes. "If we win it wouldn't be a problem so I have to accept it that maybe it's a lot."
The Spaniard added: "I always had the belief of the long season and everyone had to be involved but maybe it was too much. They played not to make mistakes instead of doing what we had to do.
"It was not the performance that we thought. I take full responsibility. We missed something. We missed an incredible opportunity and now we need to fight in the next games."
Leverkusen boss Kasper Hjulmand said of City's line-up: "It is a very tight programme for all teams. The normal rotation (number of changes) for a Champions League team is 5.5 players but no matter who they put on the pitch is a quality team."
A problem with luggage in Germany meant the plane carrying the Leverkusen team had to return to the departure gate on Monday afternoon, delaying their arrival in Manchester.
A day later, City's glut of alterations on matchday left one senior member of Bayer Leverkusen's staff - in his own words - "shocked", telling BBC Sport they had made due preparations for the contest but were not expecting that particular line-up.
Another staff member pointed to key players such as defender Edmond Tapsoba, former Real Madrid man Lucas Vazquez and Argentine duo Exequiel Palacios and Equi Fernandez being unavailable as a sign that they may struggle in the game, with six players from the under-19 side named on the bench.
Star man and goal machine Erling Haaland, Phil Foden and Rayan Cherki were all substitutes for City in the opening period and when later called upon they were ultimately unable to perform a rescue act in the second half.
Omar Marmoush failed to take his chance up front and attempted to rouse a home crowd that were often as flat as the team, while Savinho and Oscar Bobb were both on the periphery of the game.
City were slow, ponderous and the changes left them disjointed and this could end up being a damaging defeat in the competition - the loss means they head to European giants Real Madrid on 10 December under pressure to get some sort of result.
"They tried to do it [perform] but when you are in a big team you have to show off," said Guardiola. "Everyone - [including] the guys who came from the bench - were the same. Every shot was blocked, they slipped 10 times.
"Maybe with the players who played regularly lately, maybe we would have had confidence. I always like to be too nice and involve everyone because I have the feeling after the international break there are games every three or four days hopefully and there is no human being can sustain that."
Guardiola became just the third manager to take charge of 100 or more Champions League games for an English team after Sir Alex Ferguson and Arsene Wenger, but the landmark occasion was one to forget.
"The message from a lot of people will be, why didn't you play a stronger team?" asked former City midfielder Michael Brown on BBC Radio 5 Live. "Win the game and then make the changes, that is what people will say.
"There was almost an expectation that it was just going to be routine but what it did do with those changes was give that away side a massive lift. If you're walking on to the pitch looking across you'd be thinking we've got a great chance with all those players sat on the bench. That gave them the belief.
"That said, you still feel like the performance could have been a lot better from City with the players they had."
Skipper Alejandro Grimaldo's rasping strike and Patrik Schick's towering second half header gave Leverkusen a shock victory.
Led by the drummer and fan armed with a microphone, the travelling supporters and players celebrated their famous triumph together at the end.
"We are so happy with this win but also the character we showed," said Danish boss Hjulmand. "This is a night to remember and a fuel for our development.
"We did very well, played with a lot of courage and a lot of calm on the ball. We had moments where we needed luck and a very good keeper.
"Three points here at City is not something you can expect."
Former Liverpool defender Jarell Quansah added: "it's an unbelievable feeling because coming into tonight, we had a lot of senior players who were absent.
"The way the boys managed to sort of pull through – everyone led tonight and I think that's what we needed: a lot of character, a lot of determination and grit."
