London - Alfa Semedo's late goal denied Championship leaders Swansea their best ever start to a league season as Nottingham Forest beat them 1-0 on Saturday.
Swansea remain top as second-placed Charlton also slipped-up, 16-year-old Jude Bellingham scoring the only goal of the game for Birmingham.
It was Charlton's first home defeat since last October.
Fourth-placed Leeds United -- Bristol City leap-frogged them into third with a come from behind 2-1 win at 10-man Stoke -- could go top on Sunday if they beat Barnsley away.
Semedo, on loan from Portuguese giants Benfica, struck in the 85th minute to extend Sabri Lamouchi' side's unbeaten run to eight games and inflict Swansea's first league defeat of the campaign.
Swansea, who were experiencing their first league defeat, could have few complaints when Guinea-Bissau midfielder Semedo rolled home the ball from close range for his first goal for Forest.
Lamouchi, who was a surprise choice to replace the sacked Martin O'Neill in the close season, said the victory was nice but perhaps mindful he is the 13th manager of the club in eight years he was not making any wild predictions.
"I don't want to think about the future," said the 47-year-old Frenchman.
"I only know a little bit about the place and a little bit about my job, and I just enjoy it.
"I have a nice group, they trust me, and for the moment they are doing a good job.
"We deserved this victory, it was so difficult to imagine this performance before the game."
Charlton too went down to their first league defeat this season with Bellingham firing home in the 52nd minute on his full debut for the club and give his side their third win in four matches.
Charlton's disappointing day ended with their manager Lee Bowyer being sent off after he threw a loose ball towards his opponents technical area.
His Birmingham counterpart Pep Clotet was delighted Bellingham had repaid the faith he had placed in him.
"Jude is proof that we trust what we do with the academy, in that you cannot buy in football the feeling our fans have," said Clotet.
"The feeling we have for Jude to come in now, that's something we're very proud of."
Bristol City piled the pressure on Stoke's under-fire manager Nathan Jones with their win leaving him still seeking a win in 13 matches.
It had looked promising for Stoke to record their first win of the campaign when Sam Clucas fired them in front early on.
However, Welsh international Joe Allen's sending off in the 12th minute proved pivotal.
Stoke held out for 40 minutes till Famara Diedhiou equalised and then a Tom Edwards own goal sealed the points for the visitors.
Jones said he felt it was unfair they had only one point this season as their performances deserved more.
"The board sees the work I do every day, they are good football people but I don't know how long they can be patient," Jones said.
"I want to change things but nothing is going for us, there are teams with far more points than us, but are playing worse than us.
"We cant keep saying that because eventually people's patience runs out."