French Top 14 leaders Toulon and second-placed Clermont slumped to shock defeats on Saturday as the runaway title rivals came unstuck in the Basque country.
Toulon, having rested a number of top stars, suffered a 33-28 defeat away to Bayonne who avenged a 59-0 rout earlier this season.
Clermont then wasted the opportunity to take pole position by squandering a 28-22 lead at Biarritz going into the last 10 minutes before losing 32-28.
Toulon, mindful of next week's clash against champions Toulouse in Paris, left out the likes of Jonny Wilkinson and Matt Giteau while Frederic Michalak was sidelined with a shoulder injury suffered in the Six Nations.
Those absences were exploited by Bayonne who built a 20-11 lead at the interval in San Sebastian.
Long-time leaders Toulon still managed to carve out a losing bonus point, scoring three tries through Jean-Charles Orioli, Joe van Niekerk and Sebastien Tillous-Borde.
Scott Spedding, Marvin O'Connor and South African Sam Gerber, a 10th-minute replacement for winger Bastien Fuster who was taken off after a nasty clash with Toulon full-back Delon Armitage, scored tries for Bayonne.
The match was a sobering experience for Benjamin Lapeyre who was handed the task of replacing Wilkinson at fly-half and was, by his own admission, found wanting at crucial moments.
"I missed a try and some other opportunities, but some of the guys haven't had a lot of playing time," said Lapeyre, whose career has largely been spent at full-back, on the wing or in the centres.
"It's been three or four years since I played at fly-half. Everyone helped me and I have to thank the forwards. Sometimes I was a little lost but I was quick when it came to fixing my errors.
"I did some good things and some bad things, but I don't think I am going to take Jonny's place."
Clermont had been 19-13 down at the interval against Biarritz at the Stade d'Aguilera but grabbed the lead through two tries in three second-half minutes from Brock James and Davit Zirakashvili for a 28-22 advantage.
But seven minutes from time Biarritz centre Benoit Baby scored his team's second try with Dmitri Yachvili converting for a 29-28 lead which became 32-28 with a last-minute penalty.
Scrum-half Yachvili finished with 22 points while Clermont were left with the slender consolation of a losing bonus point.
Castres remained in fourth spot, and well-set for the play-offs, with a 20-9 win over Agen while Racing-Metro, in fifth, also remained in contention after a 23-19 victory against Perpignan.
Argentine full-back Juan Martin Hernandez scored Racing's only try in the first half, superbly set-up by a blind pass from winger Sireli Bobo. French fly-half Jonathan Wisniewski added six penalties.
Replacement full-back Farid Sid scored Perpignan's lone try 14 minutes from time while Welsh star James Hook kicked the conversion as well as three penalties.
At the other end of the table, Mont-de-Marsan were relegated to the second division after just one, painful season in the top flight after losing 40-7 at Bordeaux-Begles.