Ivory Coast defender Serge Aurier was sent off against his old club as Paris Saint-Germain slumped to a 2-0 defeat away at Toulouse in Ligue 1 on Friday.
Aurier was shown a second yellow card for conceding a penalty that Yann Bodiger converted early in the second half to put Toulouse in front at Le Stadium.
Jimmy Durmaz then secured the win for the home side late on as PSG suffered a second defeat in just seven league games this season to leave new coach Unai Emery facing more questions about his management.
The reigning champions took until the end of March to lose twice in the league last season but this loss followed a 3-1 reverse in Monaco last month and left Paris in third spot, a point behind Nice and Toulouse.
Leaders Nice go to Nancy on Sunday while Monaco, who host Angers on Saturday, and Bordeaux, who are at home to Caen, could both climb above PSG in the table over the course of the weekend.
"The match changed with the (first) goal and the sending-off. That was the key and after that it was more difficult for us," said Spaniard Emery, who arrived from Sevilla to replace Laurent Blanc in the summer.
"We lost the ball more than we wanted, and that is something we need to improve, but we played well in the first half and had chances to score.
"I am working to improve things but it is a long, difficult road. The team is not used to losing," added Emery, although he also said he was "optimistic".
Emery's side travelled to south-western France fresh from two wins in a week in which they had scored nine goals and conceded none to seemingly kickstart their campaign.
French international winger Hatem Ben Arfa was again left out of the squad for a fourth straight game and there was no Marco Verratti, but Aurier returned after injury.
Edinson Cavani, who had scored five times in the last two matches, was twice brilliantly denied by Toulouse's 17-year-old goalkeeper Alban Lafont in the first half before the game turned on an incident at the start of the second period.
Aurier, who had already been booked, was cautioned a second time for a foul on Ola Toivonen in the area and Bodiger sent Alphonse Areola the wrong way from the spot to open the scoring.
A young Toulouse side featured 19-year-old Clement Michelin starting a Ligue 1 game for the first time and 18-year-old loan striker Odsonne Edouard lining up against his parent club.
But they secured the points against the 10 men after a disastrous mistake by substitute Thiago Motta on 79 minutes as he gifted possession to Swedish international Durmaz, and he ran through to make it 2-0.
It was Toulouse's first win over PSG since 2009 and they celebrated with a lap of honour on the pitch at full-time.
"These are young players and they have above average technical ability, but they find it difficult to let themselves go," said Toulouse coach Pascal Dupraz, who led his team to a miraculous escape from relegation at the end of last season.
"I hope this win helps the players perform for the whole game from now on. We need to continue like this."