Borussia Dortmund fell to a shock defeat as they were beaten 3-1 by Hamburg in the Bundesliga on Friday.
Pierre-Michel Lasogga scored a penalty to give the home side a first-half lead after Ivo Ilicevic had been brought down by goalkeeper Roman Burki.
Lewis Holtby made it two just before half-time and a bizarre Mats Hummels own goal in the second half ended any chance of a comeback for Thomas Tuchel's side.
Pierre-Emerick Aubameyang scored his 15th league goal of the season with four minutes remaining, but by then the damage was done.
Dortmund had come into the game on the back of four straight Bundesliga victories, but the surprise loss - only their second of the season in the top flight - means leaders Bayern Munich now have the chance to move eight points clear when they play at Schalke on Saturday.
Hamburg, meanwhile, won for only the second time in their last seven league matches to move up to eighth in the table.
The visitors made only one change, with Marco Reus returning from injury to replace Gonzalo Castro after kick-off was delayed by 15 minutes to allow for extra security checks, with Germany's friendly against Netherlands having been cancelled on Tuesday.
Hosts Hamburg created the first decent opening when Lasogga chested down a cross from the left to set up Ilicevic, who sliced his shot wide badly wide.
Ilicevic had more success soon after, though, winning a 19th-minute penalty. A throughball from Holtby sent the Croatian racing through on goal and he went to ground after colliding with Burki as he tried to round the goalkeeper, who was given a yellow card.
Lasogga took the resulting spot-kick and convincingly found the far corner despite Burki guessing the right way.
Dortmund had most of the possession but were struggling to carve out clear-cut chances, with Rene Adler pushing away a scuffed Reus effort on one of the few occasions the visitors were able to threaten.
Things got even better for Hamburg four minutes before half-time. A loose pass from Matthias Ginter set up a break for the hosts, with Nicolai Muller patiently holding the ball before playing in an onrushing Holtby at the right moment, with the midfielder doing the rest by calmly slotting his finish into the far corner.
Tuchel made a double change at the interval, with Ginter and Shinji Kagawa being replaced by Castro and Lukasz Piszczek, before Marcel Schmelzer, Henrikh Mkhitaryan and Aubameyang all failed to find the target with decent shooting opportunities early in the second half.
Those misses proved costly when Hummels scored a disastrous own goal ofter 55 minutes. Under little pressure in the box, the Germany defender headed Holtby's corner down into the ground and could only watch on as it bounced past Burki.
A consolation goal arrived for Dortmund when Mkhitaryan linked up nicely with substitute Adnan Januzaj and the on-loan Manchester United playmaker fed Aubameyang, who slotted past Adler.
Adler made excellent saves from Castro and Aubameyang, before the Gabon striker had the ball in the net in stoppage time. A rasping drive from Piszczek struck the post and Aubameyang converted the rebound, only to be denied by a controversial offside decision.