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In the fog that drifted over the City Ground, Enzo Maresca's side were all at sea as they slid to their third league defeat in as many games.
Nottingham Forest had bottle and bite right from kick-off while Manchester City were brittle and bumbling.
City have now conceded eight first-half goals in 10 games, with only Crystal Palace (11) and Ipswich (nine) conceding more in the league this season.
The scrutiny on City goalkeeper Gianluigi Donnarumma, who has suffered a dip in form in recent weeks, will inevitably increase.
A well-placed totem pole might have been more effective than the Italian for Forest's first two goals.
On both occasions the 27-year-old's positioning and reaction times were questionable.
He was not helped by those in front of him, as decidedly meek play from Elliot Anderson and Josko Gvardiol led to Forest's opening two goals.
Indeed, only Sunderland (six) and Hull City (five) have committed more errors leading to goals in the Premier League this season than City's four.
There was little Donnarumma could do about the quality of Liam Delap's finish for the third.
Maresca needs to eradicate such sloppiness if City are to keep pace with a ruthless Arsenal side deep into the season.
At the moment it looks unlikely.