Bayern Munich spurned a host of chances as Leon Bailey's double earned 10-man Bayer Leverkusen a 2-1 win and inflicted a first defeat on Hansi Flick.
The champions had won each of their four matches without conceding since Flick was named interim boss following Niko Kovac's sacking, but they came unstuck in Saturday's Bundesliga clash at the Allianz Arena.
Thomas Muller scored his first goal in 21 league matches to cancel out Bailey's early opener, only for the Leverkusen winger to restore his side's advantage 115 seconds later.
Despite Jonathan Tah being dismissed nine minutes from time, a mixture of poor finishing and impressive goalkeeping from Lukas Hradecky left Bayern – who rattled the woodwork late on through Leon Goretzka and an unusually wasteful Robert Lewandowski – frustrated as they lost further ground in the title race.
Serge Gnabry hit the post after being played in by an incisive David Alaba pass and, 45 seconds later, Bailey charged through at the other end before firing a shot across Manuel Neuer to open the scoring.
Some heroic defending from Wendell denied Lewandowski a simple tap-in but Bayern were soon on level terms, Muller's shot deflecting off Sven Bender on its way past Lukas Hradecky.
The visitors instantly hit the front again through Bailey, though. The winger played a one-two with Kevin Volland, raced away from Javi Martinez and slotted past Neuer to double his tally.
Gnabry squandered a glorious chance to draw Bayern level late in the first half as he squared the ball instead of shooting when one-on-one, with Lars Bender blocking the pass.
Neuer was required early in the second period to keep out Nadiem Amiri's close-range shot, but it was Flick's men who were enjoying the lion's share of chances.
Lewandowski was wayward from 10 yards and Hradecky produced a double save to thwart the Poland international and Muller in quick succession.
Goretzka and Lewandowski both sent headers crashing against the woodwork, either side of Tah being shown a red card for bringing down substitute Philippe Coutinho, but there was to be no equaliser for the champions.
What does it mean? Reality check for Bayern
Emphatic back-to-back Bundesliga victories for Bayern had them firmly back in the title race, with talk of caretaker boss Flick potentially landing the job on a full-time basis.
This defeat is very much a reality check for the Bavarian club, however, leaving them fourth in the table and now three points behind RB Leipzig, who beat Paderborn 3-2 earlier on Saturday, while if Borussia Monchengladbach defeat Freiburg on Sunday they will be four off the pace.
Bayern battered by Bailey
Bailey had gone six matches without a goal prior to this trip to Bavaria but looked at his scintillating best in the first half.
Bayern were unable to contain the winger, who put his pace to good use and twice kept his cool when through on goal.
Lewandowski kept quiet
Lewandowski scored at least once in 11 consecutive Bundesliga matches, but he has now fired blanks in back-to-back league matches.
The Poland international missed a one-on-one in the first half, though it would not have counted as he was offside, and looked off the pace thereafter - bar a brilliant late header that was unfortunate to hit the bar.
Bayern return to action next Saturday with a trip to high-flying Borussia Monchengladbach, while Leverkusen host Schalke on the same day.