0150 GMT September 26, 2017
Bayern never recovered from the shock of falling behind to the first of Mark Uth's two goals on 27 minutes, badly caught napping following a quickly taken throw-in from Andrej Kramaric (via a ball boy) which meant two balls were on the pitch for a brief moment, ESPN reported.
Speaking after the match, Ancelotti blamed his side's profligacy in front of goal for the disappointing result.
"Little details decided the game, for the first goal we weren't focused and conceded a second goal immediately in the second half. It was difficult then but we had a lot of chances.
"We controlled the game but didn't score while Hoffenheim was three times in our box and scored two goals," he said.
"In general, we played well but we weren't at 100 percent – 95 percent wasn't enough, but that happens in football and we have to focus on the next game."
Neuer echoed his coach's sentiments, but added that "We began the game very well and had Hoffenheim totally under control. We had a few chances and should have taken the lead,"
Commenting on the opening goal, "It all happened very quickly but basically we weren't a man down and weren't ideally positioned.
"That was our mistake and did not have anything to do with the second ball; you simply have to win the challenge. Then Uth doesn't connect perfectly with his shot and the ball goes in at the near post," Neuer said.
"I don't really want to talk about the time after. We had Hoffenheim completely under control and should have taken the lead here. That's the problem."
Last season Hoffenheim took four points from its two Bundesliga games against Bayern, with the 1-0 home win in April marking its first ever victory in 18 meetings.
Hoffenheim coach Julian Nagelsmann said, "We are delighted that we have won – we were worse in every statistic, except for the goals.
"Luck was also on our side, but we have worked hard and proved we have a great heart.''
Bayern hosts Anderlecht in its opening Champions League fixture on Tuesday.