An 88th minute strike from Troyes' Benjamin Nivet handed Marseille their second defeat of the Ligue 1 season, 1-0, and left PSG top of the table on goal difference.
Marseille had much the better of the match, despite a serious injury to striker Andre-Pierre Gignac, but Nivet arrived to find a winner with Troyes' first shot on target with only minutes to go.
After the game, Marseille boss Elie Baup told the club's official website of his striker: "This is more bad news, but we don't know how long he will be out for yet.'' While Gignac added: "This is not good news. It's part and parcel of being a footballer, but I often have bad luck.''
Earlier, Bafetimbi Gomis fired Lyon into the top three with the only goal of the game against Brest at the Stade Gerland.
Gomis struck just before the hour mark to end Lyon's three-game winless run and keep them in touch with the leaders.
While it was hardly the most convincing performance from Remi Garde's side, against a Brest team that has now lost all five of their away games, it was a much-needed success ahead of next weekend's anticipated trip to leaders Marseille.
Both sides had a man sent off as Ajaccio and Bastia shared a bad-tempered goalless draw in the Corsican derby.
Bastia defender Gael Angoula and Ajaccio forward Johan Cavalli were given straight red cards eight minutes from time following a melee involving players from both teams. The ill-feeling spread to the stands with the fans getting fired up and setting off flares.
Information from the Press Association was used in this report