Milivoje Novakovic scored twice as Cologne came from two goals down to beat Bayern Munich 3-2 at home in the Bundesliga on Saturday.
Bayern drops to fifth, 15 points behind leader Borussia Dortmund, overtaken by Mainz and Hannover in third and fourth respectively. Cologne remains 16th.
"They must be slowly laughing themsevles to death in Dortmund," said Bayern CEO Karl-Heinz Rummenigge.
Bayer Leverkusen lost 1-0 at Nuremberg but remains second, 12 points adrift of Dortmund.
In Saturday's late game between the bottom sides, a late Timo Gebhart penalty gave Stuttgart a 3-2 win at Borussia Moenchengladbach to complete a memorable comeback.
Dante, a defender, and Igor de Camargo gave Moenchengladbach a 2-0 halftime lead, but Pavel Pogrebnyak scored in the 51st, and Martin Harnik scooped the ball over the goalkeeper for the equalizer five minutes later.
Gebhart scored from the penalty spot in the 87th, after Dante was adjudged to have brough Pogrebnyak down and sent off for a second yellow card.
Moenchengladbach remains bottom, six points from safety and three behind Stuttgart.
Elsewhere, Claudio Pizarro scored an injury time goal for Werder Bremen to earn a 1-1 draw at Mainz, while there were home wins for Hannover and Hoffenheim over Wolfsburg and Kaiserslautern respectively.
"The decisive factor was how we believed in ourselves in the second half," Cologne coach Frank Schaefer said. "We played with too much fear in the first."
Bayern should have been reduced to 10 men in the 19th minute, when Holger Badstuber, the last defender, cynically brought down Novakovic when the striker was through on goal.
The Bundesliga's youngest referee, Felix Zwayer, showed Badstuber a yellow card.
Gomez scored his 16th of the season when he deflected a cross from Thomas Mueller past former teammate Michael Rensing. The Cologne goalkeeper was unlucky to see the shot deflect in between his legs.
Bayern - playing without Arjen Robben due to a cold - had full control of the game, and it was just a matter of time before the second arrived.
It came in the 43rd when Hamit Altintop blasted the ball past Rensing into the far corner from a tight angle.
"We saw a very different Cologne in the second half," Lukas Podolski said. "The way we turned the game around with the fans behind us was simply sensational."
Clemens scored when he ran onto a through ball from Christian Eichner and rounded goalkeeper Thomas Kraft to finish from a tight angle.
Novakovic's equalizer prompted Bayern coach Luis van Gaal to bring on Franck Ribery for his first Bundesliga action since Jan. 15th.
But Novakovic scored again when he held off the challenge of Badstuber and blasted it past Kraft to leave the goalkeeper shaking his head.
Clemens should have added another for Cologne late on, but he blasted the effort over.
"We made life difficult for ourselves in the second half," Bayern captain Philipp Lahm said. "We simply made too many mistakes. We didn't perform again in the beginning of the second half. We've given away very important points. That's what's bitter, that's what hurts."
In the Lower Saxony derby, Steve McClaren's new-look Wolfsburg side had a bad start at Hannover when Sergio Pinto scored in the fourth minute. The midfielder let fly from 23 meters (yards) and Diego Benaglio somehow let it slip under his arms.
Diego crashed a penalty off the crossbar in the 80th, after he was brought down in the area by Emanuel Pogatetz. It was Wolfsburg's third penalty miss in a row, and Hannover held on to win 1-0.
Werder Bremen's Claudio Pizarro scored in the third minute of injury time, when he turned smartly in the area to volley past Mainz keeper Heinz Mueller.
Andre Schuerrle scored for Mainz in the 19th, and dedicated the goal to teammate Adam Szalai, ruled out for the rest of the season because of a right knee injury.
Christian Eigler scored the winner for Nuremberg over Bayer Leverkusen, before he was sent off late on for a second yellow card.
Gylfi-Thor Sigurdsson and Sebastian Rudy gave Hoffenheim a 2-0 halftime lead over Kaiserslautern, but the visitors equalized with two goals in as many minutes from Erwin Hoffer and Brazilian defender Rodnei in the 58th and 60th.
Vedad Ibisevic scored the winner for Hoffenheim in the 62nd to make it 3-2.
