Leaders Clermont-Auvergne suffered a major blow to their Top 14 title aspirations as they slipped to a shock 22-14 defeat at bottom side Bayonne.
The Basque side had recorded only one league victory all season heading into the game, but led 13-7 at the break courtesy of Jean Monribot's converted try and two penalties from Manu Saubusse.
Saubusse kicked a further nine points in the second half, helping to cancel out Benson Stanley's 68th-minute try for the visitors.
Montpellier climbed to second as they bounced back from last week's defeat at Racing 92 by defeating Lyon 25-20.
Full-back Joffrey Michel's converted try six minutes from time finally sealed the victory for Jake White's men, who had earlier conceded five-pointers to Felix Lambey, Carl Fearns and Taiasina Tuifua.
Lyon's cause was not helped by the loss of South African back-rower Deon Fourie to a yellow card on the stroke of half time.
Tries from Yann Lesgourgues and Nans Ducuing helped Bordeaux-Begles to move into the top four as they earned a hard-fought, 24-22 win at struggling Grenoble.
The home side actually outscored the visitors in terms of tries, with Gio Aplon, Chris Farrell and Lilian Saseras all finding the line, but they still slumped to their ninth defeat of the campaign.
Castres, meanwhile, secured a convincing 32-13 victory in their mid-table battle with Brive at the Stade Pierre-Antoine.
Alex Tulou and Jody Jenneker crossed either side of the break for the hosts, while Argentinean fly-half Benjamin Urdapilleta added a further 17 points with the boot.