Folarin Balogun missed two penalties on his first Monaco start before local rivals Nice grabbed a stoppage time winner to go top of Ligue 1 on Friday.
One week after stunning champions PSG at the Parc des Princes 3-2, Nice claimed the three points again when Jeremie Boga slalomed through the Monaco defence to score ans Francesco Farioli's team continued an undefeated start to the season.
Nice leapfrogged previously unbeaten Monaco to take top spot by one point after a third win in six games.
It was a miserable first start for US international striker Balogun who left Arsenal for Monaco in the summer on a 30-million-euro transfer.
He had penalties saved by Nice goalkeeper Marcin Bulka in the 12th and 54th minutes.
His first effort, awarded when Jordan Lotomba brought down Aleksandr Golovin, was tame.
His second spot-kick, which came after Morgan Sanson was penalised for a foul on Takumi Minamino, was again saved by the Polish goalkeeper.
Balogun, who had scored off the bench in last week's 2-2 draw with Lorient, was eventually replaced by Wissam Ben Yedder, a penalty-taking specialist.
