Defending champions Racing 92 made a stuttering start to their Top 14 title defence as they were beaten 15-9 by Bordeaux Begles on Saturday.
Less than two months after Racing secured a 29-21 victory over Toulon in the final in Barcelona, they were found wanting in Bordeaux, the boot of Ian Madigan proving decisive for the hosts.
The Ireland international kicked five penalties to secure Bordeaux's victory, three efforts from Dan Carter all the reigning champions could muster.
Racing pushed hard in the latter stages and broke through on a couple of occasions, but Bordeaux's defence remained resolute as they clung on to a memorable victory.
Clermont Auvergne - who topped the table at the end of the regular season in 2015-16 - were also denied victory on the opening day as La Rochelle produced a second-half comeback to draw 30-30.
Noa Nakaitaci, Arthur Iturria and David Strettle all scored in a dominant first half for Clermont but they could not turn their 24-17 half-time lead into a win, Brock James kicking a late penalty to earn La Rochelle a share of the spoils.
Newly promoted Lyon also started with a draw as they held Brive 15-15, but elsewhere there were big victories.
Stade Francais ran in five tries as they beat Grenoble 54-20, while Castres were too good for Pau, winning 28-11.
Saturday's final match saw Toulouse score 10 points in each half to beat Montpellier 20-12, Sebastien Bezy top scoring with two penalties and two conversions.