Manchester City slumped to a third consecutive Premier League defeat on Saturday as Liam Delap's double lifted Nottingham Forest out of the relegation zone with a 3-1 win at the City Ground.
Oliver Glasner's men had not scored at home at all this season but pounced on City's defensive errors to strike twice in the first 20 minutes through Jair Cunha and Delap.
Rayan Cherki's stunning strike gave City hope, but their title challenge is rapidly wilting even before the potential effects of a major points deduction.
Delap sealed just Forest's second win of the season on the night that could launch his career for the two-time European champions, after a difficult start since his club-record £50m move from Chelsea.
Defeat leaves City six points adrift of leaders Arsenal and with questions over whether the uncertainty surrounding the club is playing a part in their on-field slump.
Found guilty on more than 100 Premier League charges of breaking financial rules, the 10-time English champions face a severe points penalty unless they can overturn that verdict on appeal.
"It's quite clear, first half we were not good enough to match the intensity that this kind of games require," City boss Enzo Maresca said.
"We concede two and then we try to react, but most of the time in football when you are 2-0 down it's too late."
Glasner warned his players he would make big changes after a disappointing start to his reign saw them start the match in the bottom three.
The Austrian got the response he craved as Forest burst out of the blocks to pounce on City's sloppiness at the back.
"I think everybody felt huge happiness but also relief," Glasner said.
"It can be a turning point but we have to prove it."
Cunha robbed Josko Gvardiol midway inside the City half and was allowed to bear down on goal before slotting past Gianluigi Donnarumma.
Forest's first home goal in 511 minutes was celebrated wildly with owner Evangelos Marinakis jumping out of his chair in celebration.
Elliott Anderson had a night to forget on his first return to the City Ground since a reported £116m move to City.
Xaver Schlager beat Anderson to the ball in midfield and set Delap running from inside his own half before unleashing an unstoppable drive past Donnarumma for his first home goal for the club.
Maresca became the first Premier League manager to win his first seven games in charge of a new club in a blistering start to the daunting task of succeeding Pep Guardiola.
However, he has been hit with the task of trying to keep his players motivated when they do not know if they will even be in the English top-flight next season.
Maresca showed his frustration with a double change at half-time with the introduction of Nico O'Reilly and Matheus Nunes helping to spark a brief City rally.
Cherki smashed home a sweet strike from Jeremy Doku's deflected cross to halve the arrears.
Erling Haaland then nearly levelled when the Norwegian flashed a shot just wide.
City, though, remained vulnerable to the Forest counter-attack and were picked off again for the decisive goal 15 minutes from time.
Morgan Gibbs-White fed Delap, who drilled another brilliant finish past the helpless Donnarumma, this time on his left foot.
