Clermont Auvergne could only draw with lowly Grenoble and Toulouse took advantage by putting Bordeaux-Begles to the sword in the Top 14.
Three-time European champions Toulon have been struggling in the Top 14 and lost to La Rochelle on Sunday.
Connacht had no answer to Chris Ashton, who enjoyed a hat-trick on his competitive debut for Sale Sharks.
Clermont's advantage at the summit of the Top 14 table was reduced to four points after they were beaten at La Rochelle.
Top 14 leaders Clermont Auvergne put some daylight between themselves and second-place Stade Francais on Saturday.
Clermont Auvergne remain top of the table, but they suffered a first defeat of the Top 14 campaign at the hands of Bordeaux.
Greig Laidlaw kept his cool to maintain Clermont Auvergne's perfect start to the new Top 14 season.
Clermont won an early top-of-the-table clash with Stade Francais, while there were also Top 14 victories for Racing 92, Toulouse and Lyon.
Castres overturned a 13-point deficit in the second half to beat Lyon in Saturday's Top 14 action.
Clermont Auvergne ran hot at the start of the new Top 14 season as the two promoted sides came up short in their opening outings.
Ahead of the opening round of fixtures in the 2018-19 season, we pick out some of the best statistics surrounding the Top 14.
Racing 92, Lyon and Castres got the play-off outcomes they were hoping for in Top 14, while Stade Francais stay up despite suffering defeat.
Racing 92's bonus-point win over Bordeaux-Begles gives them the slenderest of advantages over Toulouse with one match to play.
Former England wing Chris Ashton scored a record 22nd Top 14 try of an outstanding debut season as Toulon beat the leaders at Stade Mayol.
Castres, Stade Francais, Lyon, Bordeaux-Begles and Agen were the winners on a busy day of Top 14 action.
Montpellier ran in five tries to claim a bonus-point victory over Castres, maintaining their position as Top 14 leaders.