Paris Saint-Germain academy product Jonathan Ikone came back to haunt his old club on Monday as he scored the only goal in Paris FC's 1-0 win over their city rivals to reach the last 16 of the French Cup.
Substitute Ikone stabbed home in the final 20 minutes to surprise the dominant record 16-time winners and current champions at the Parc des Princes.
PSG lost their first home game in the competition since 2022 and suffered their first last-32 defeat since 2014.
"We're really happy, we managed to defend well," Ikone told France Televisions.
"I'm really happy with my goal, just a joy, and I hope it's not my last goal," he added.
Before kick-off the hosts' former captain Mamadou Sakho, who began his career with the Paris FC academy before crossing town, announced his retirement on the pitch.
The sides met earlier this month in the first top-flight derby in the City of Light since 1990, as Desire Doue and Ousmane Dembele scored in a 2-1 win for the Champions League holders.
Since then, PSG have lifted the French Champions Trophy edging rivals Marseille on penalties in Kuwait last Thursday.
Newly-promoted Paris (PFC), backed by luxury goods brand LVMH and Red Bull, are 15th in Ligue 1, two points above Nantes in the relegation play-off berth.
Luis Enrique's home side dominated the first half, with Georgia's Khvicha Kvaratskhelia threatening without breaking the deadlock.
PFC winger Alimami Gory had the best early chance for the visitors before being substituted by Ikone five minutes before the interval due to injury.
Ten minutes into the second-half a sloppy pass from PFC centre-back Moustapha Mbow handed PSG a golden chance to score but goalkeeper Obed Nkambadio punched away Goncalo Ramos' curled effort.
With PSG still on top with 15 minutes to play, PFC broke on the counter before attacker Ikone poked home to make it 1-0 for his side before refusing to celebrate.
During the seven minutes of injury time Doue came within inches of the equaliser with a header before Nkambadio made another superb save to cap off a stellar individual performance to stop a long-range Vitinha strike.
In the final game of the round, Marseille head to sixth-tier Bayeux with the game moved elsewhere in Normandy in front of a full-house at Caen's Stade Michel-d'Ornano.
Bayeux, the home of the centuries-old tapestry, are the lowest team remaining in the tournament with the draw for the last 16 taking place before kick-off in northern France.
On Saturday, former Wolves coach Gary O'Neil started his tenure as Strasbourg boss with a 6-0 win over fourth-tier Avranches.