Franck Ribery was left stunned as Bayern Munich surrendered a two-goal lead to lose 3-2 at home to Manchester City on Tuesday.
Strikes from Thomas Muller and Mario Gotze in the opening 12 minutes put the holders in complete control, but die Roten capitulated to allow the Premier League side to secure the win thanks to David Silva, an Aleksandar Kolarov penalty and James Milner.
"I honestly do not know what happened," Ribery told BeIn Sport. "We started well in the game, but after that we took this team lightly. There was a loosening up and ultimately the game is lost.
"We are pleased with first place [in the group], but upset regarding the result. We made a lot of mistakes due to our lack of concentration. We hope to respond this weekend."
Captain Philipp Lahm admitted his side were fortunate to keep their place at the top of the group on a narrow head-to-head record after their poor second-half showing at the Allianz Arena.
"If you are two goals up front you need to play it better than today," he told Sky Deutschland. "The good thing is that we can live with the loss because we are first in the group. You would rather win, but if you lose it should be a loss like this."
Both Muller and defender Dante were left aggrieved at the decision to award a spot-kick following the Brazilian's trip on Milner.
"It was a bit strange in the second half. I don't know if that was a penalty. In the end we have to be satisfied that it was enough for winning the group," said Muller, before Dante added: "I was sure that was not a penalty."