Clermont Auvergne, unbeaten at home for 77 games before Castres shocked them in the play-offs at the end of last season, fell to another defeat at Stade Marcel Michelin at the weekend when Montpellier secured their first ever win there with a 21-20 success.
French out-half Francois Trinh-Duc landed a last-gasp penalty to condemn Clermont to their first defeat of the season.
No 8 Fritz Lee, the Samoan-born New Zealand sevens player, scored the only try of the game as Clermont are still waiting for one of their expensive backline to cross the whitewash this season.
Clermont entertain Racing Metro tomorrow when they will hope to make the most of Jonathan Sexton's absence through injury. Munster's other two pool opponents swing into competitive action this weekend. Saracens take on Wasps in part of the Twickenham double-header, with London Irish and Harlequins facing off in the other tie.
Sarries completed their pre-season campaign with a 100pc winning record from three matches when they won 47-3 away to Bedford Blues.
They scored seven tries, two of them penalty tries, as their pack were on top throughout against the home side.
Sale Sharks were not in action at the weekend and will open their Premiership campaign against Bath at AJ Bell Stadium, the first of three home matches before entertaining Munster on October 18.