Cesc Fabregas scored a late winner from the penalty spot as Monaco came from two goals down to stun reigning champions Paris Saint-Germain 3-2 in Ligue 1 on Friday.
Kylian Mbappe had netted a first-half brace, including a penalty, against his former side, taking him to 99 goals for PSG and leaving his current team seemingly on course for a ninth consecutive league win.
But German international Kevin Volland struck twice for Monaco after the break and Fabregas converted from 12 yards in the 84th minute after Abdou Diallo had been sent off for conceding the all-important penalty.
PSG remain clear at the top of Ligue 1, but this result is a major setback for Thomas Tuchel's team before a crucial Champions League tie against RB Leipzig next week.
"I am more surprised than I am angry," said Tuchel, who was also disappointed to see further first-half goals for Mbappe and Moise Kean disallowed.
"I saw two totally different halves. First of all, we completely controlled the game…and then the second half was completely different. I can only say it is our responsibility."
Niko Kovac's Monaco climbs up to second in the table, four points behind PSG, but Lille can reclaim the second spot and move to within two points of the leaders if they beat Lorient on Sunday.
"If you come back from two down to win 3-2 it is for sure something special, but I don't want to glorify it too much," said ex-Bayern Munich coach Kovac.
"We cannot stop and say everything is done. It was a big victory but nothing more."
PSG have been severely handicapped by injuries and suspensions this season, and Neymar was only deemed fit enough for a place on the bench at kick-off at the Stade Louis II after missing Brazil's World Cup qualifiers over the last week due to fitness problems.