Just 10 days after thrashing Manchester United, City were taught a footballing lesson themselves as the European champions powered to an impressive 3-1 win at the Etihad Stadium.
The vaunted attacking trio of Franck Ribery, Thomas Mueller and Arjen Robben all got on the scoresheet in a masterful display but, as brilliant as the Germans were, City contributed to their own downfall.
The performance of Hart will come under the most scrutiny after the England goalkeeper made a disappointing attempt to keep out Ribery's strike and was beaten too easily at his near post by Robben.
A run of five successive clean sheets through August and September had taken some of the heat off Hart, who has made a number of high-profile errors in the past year, but the vulnerabilities remain.
He was not the only culprit, however, with Gael Clichy losing Muller for the second goal and Fernandinho gifting possession too easily in the build-up to the third.
The consolation for the English side came from substitute Alvaro Negredo 11 minutes from time while David Silva also hit the bar in a strong finish after former City defender Jerome Boateng was sent off.
Despite that, the overwhelming impression was one of City being outclassed by a superior side.
Pep Guardiola's side played without an out-and-out striker but swamped their hosts in midfield and broke with pace and incisive passing.
Robben bluntly summed up the difference between the two sides.
The Dutchman told Sky Sports: "To be so dominant against such a big team, we didn't expect an easy game like that.
"I think it's also a big compliment to our team but I expected a bit more from City."
Guardiola, who has remarkably faced criticism despite an unbeaten start to the Bundesliga season, was a bit more measured in his assessment of City.
The former Barcelona boss said: "I am respectful when you play against Manchester City with these unbelievable players.
"We played better, we won, but the football was special.
"It is nice for the team to come to this country, where they love football so much, and play like we did."
The result took Bayern clear at the top of Group D and, even after just two games, City now look to be in a battle with their next opponents CSKA Moscow for second place.
Pellegrini did not want to single out Hart for criticism but did admit concern.
He said: "Of course I am worried about the mistakes but I think that it was not an easy game, we played against a very strong team.
"Our performance was not the way we were playing. It is important to think about the future and what are the solutions."