Substitute Valentin Lavigne climbed off the bench to fire home a late winner as Lorient stunned 10-man Monaco 2-1 to claim their first-ever win at the Stade Louis II.
Monaco's star striker Radamel Falcao, who also started on the bench, had looked to have earned last season's runners-up a point when he slotted home a 78th-minute penalty to cancel out Vincent Aboubakar's early spot-kick for Lorient.
However, it was another replacement who ended up making the decisive impact as 20-year-old Lavigne took advantage of the wide open spaces behind the Monaco defence to put his side back in front in the 87th minute.
The hosts, who had captain Ricardo Carvalho sent off midway through the second half, could not fashion a second equaliser after that as they slumped to a shock defeat in new coach Leonardo Jardim's Ligue 1 debut.
Jardim took over from Claudio Ranieri this summer and enjoyed a good pre-season with the Monegasque, winning five and drawing the other of their six friendlies, but they found things much tougher in their competitive bow on Sunday.
It took visiting Lorient just nine minutes to take the lead when Monaco defender Aymen Abdennour swept away Aboubacar's standing foot inside the area to concede a penalty, which the Lorient striker confidently slotted home with Danijel Subasic going the wrong way.
It took shellshocked Monaco a while to threaten an equaliser, and when they did create an opening Dimitar Berbatov's downward header was pushed out by Benjamin Lecomte and the Lorient defence hacked clear before Joao Moutinho could get to the loose ball.
Lecomte might have done a little better on that occasion, but the 23-year-old was to prove a real thorn in Monaco's side by the end of the game.
He denied Layvin Kurzawa before the break and then produced vital saves to keep out efforts from Berbatov and Yannick Carrasco at the start of the second half.
Monaco then lost veteran defender Carvalho, who was shown a second yellow card for a body check on Aboubakar, but the home side continued to create chances with Lecomte producing a stunning finger-tip save to turn behind another Berbatov header.
Berbatov was also involved in the penalty incident that led to Monaco's equaliser, the referee deeming Lamine Kone's challenge from behind on the Bulgarian illegal.
Lorient may have felt it was a harsh decision but Falcao, who only recently returned to action following a serious knee injury, took little sympathy as he fired his penalty high to Lecomte's right as the keeper dived in vain to stop it.
Monaco may have hoped to push on for the winner after that, but instead it was Lorient celebrating the three points as Lavigne, who had only been on the pitch six minutes, was put through on a quick counter-attack following a mistake by Abdennour and the youngster coolly slotted past Lecomte to seal an historic win for the visitors.