Jeremie Aliadiere scored the winner for Lorient as they beat Saint-Etienne 2-1 in the Coupe de France quarter-finals at the Stade Geoffroy-Guichard.
ASSE will meet Rennes in the Coupe de la Ligue final in Paris at the weekend and had designs on reaching the last four of this competition after piecing a 17-game unbeaten run together in all competitions.
That stunning streak - which has them on the verge of Champions League qualification in the league - was made shaky when Maxime Barthelme opened the scored just before half-time.
St Etienne's response only came after star striker Pierre-Emerick Aubameyang stepped off the bench just after the hour and scored with 15 minutes left.
But rather than kick on for victory Lorient stunned their hosts as former Arsenal frontman Aliadiere stole the win in time added on.
In Tuesday night's other match Troyes put their woeful league form to one side to beat fellow relegation rivals Nancy 3-0.
All the goals arrived after the break as Jeremie Brechet (51), Julien Faussurier (74) and substitute Fabien Camus (81) finally gave Troyes, who are rock bottom in Ligue 1, some reason for cheer.