A dropped pass by Aurelien Rougerie in the dying seconds with the try-line beckoning robbed Montferrand of a last-gasp victory and a money-spinning Heineken Cup semi-final tie.
The French knew they would have one team in the semis, but it took a Scotsman, stand-off Gregor Townsend, to put Castres on their way to a 22-21 victory.
Townsend landed an early drop goal, but his team-mates were left to sweat as Montferrand battled back bravely into a match the home side thought they had in the bag.
The opening period was largely dominated by the kicking of Romain Teulet and Gerald Merceron. The pair swapped two penalties before the home side edged in front with a third from the prolific Teulet.
Townsend made his contribution to send his team further clear and Teulet put the home side 15-9 up at the break with a late first half penalty. But Montferrand began the second half in determined fashion and two quick penalties from Merceron drew the sides level.
Froment then bundled over the line following a scrum five yards out for the game's solitary try. Merceron notched two further penalties to bring Montferrand to within one point of their hosts before Rougerie's agonising handling error ensured Castres advanced to the last four.