Manchester City's Premier League title hopes were hit after losing 1-0 at Everton from a goal by former Man United midfielder Darron Gibson on Tuesday.
Gibson's first goal since leaving Old Trafford in the January transfer window via a deflection off Gareth Barry on the hour consigned City to just its third loss of the league campaign, and boosted the title hopes of his old team.
"A goal against any club is good," said Gibson, whose strike was set up by United States midfielder Landon Donovan. "A goal against top of the league is great."
City is now only top on goal difference after United beat Stoke 2-0, while Everton climbed to ninth in the standings.
"I didn't prepare well for this game. I thought it was maybe easier," City manager Roberto Mancini said. "I know here it is never easy, never. I acknowledge my mistake. It is important for me to know this because, like this, I can't do the same mistakes in the next game."
When asked to elaborate on his mistakes, Mancini referred to the team's recent training sessions.
"The players put everything on the pitch but I made some mistakes during the last three days preparing for this game," the Italian coach said.
Everton manager David Moyes felt his side's victory was one of the best he had been involved with in his near 10 years at the club.
"That is as good as it has been at Everton for a long time," he said. "With the team we had out, the players we had missing, it says a lot about the club and how we always seem to come back to do it.
"We might have played smoother football and been better, but against the side we were playing against tonight, the players deserve a lot of credit. Man City are a terrific team. We had to defend for long periods, especially in the second half."
That second half started later that expected. The end of the first period was interrupted for several minutes when a supporter handcuffed himself to a goal post. Police officers took several minutes to release the supporter.
After the match, Everton managed to re-resign midfielder Steven Pienaar on loan from Tottenham until the end of the season a year after leaving, while striker Louis Saha moved in the opposition direction on a permanent deal.
Everton also strengthened its scoring options by signing Croatia striker Nikica Jelavic from Rangers.