Guingamp came from behind to draw 2-2 with 10-man Monaco before clinching a dramatic victory on penalties in Tuesday's Coupe de la Ligue semi-final, as Leonardo Jardim endured a disappointing return to the Principality club.
Monaco, who appointed Jardim for his second spell as coach after sacking Thierry Henry on Friday, raced into a 2-0 lead despite having loanee midfielder William Vainqueur sent off.
Rony Lopes and Aleksandr Golovin found the net for the visitors but Guingamp improved dramatically in the second half and levelled through Alexandre Mendy and Marcus Thuram.
Marcus Coco held his nerve to score the winning penalty for Guingamp as Monaco's nightmare season continues, and Jardim must now pick his players up before they face Toulouse in Ligue 1 on Saturday.
Vainqueur was booked for a two-footed challenge on Thuram after 10 minutes but the referee consulted VAR before producing a red card and sending the former Roma playmaker off.
But eight minutes later Monaco were ahead when sloppy Guingamp defending allowed Gelson Martins to maraud into the box before squaring through a crowd of players to Lopes, who had time to control the ball, flick it up and acrobatically volley it over his shoulder into the bottom-left corner.
The 10 men doubled their advantage when Lopes launched a fast break from his own half and Martins burst away before teeing up Golovin, who flashed a shot high into the net inside the near post.
The second half had barely begun when Mendy took advantage of a loose ball and slotted it low beyond Danijel Subasic from inside the box, and the equaliser followed 10 minutes later when the superb Thuram turned Martins and hooked a superb finish into the top-right corner from 20 yards.
Guingamp attacked for much of the remaining 35 minutes but Monaco held on for a shoot-out that saw substitute Sofiane Diop hit the crossbar, allowing Coco to step up for the winner and prompt scenes of jubilation around the Stade de Roudourou.