French Top 14 leaders Toulon, having rested a number of top stars, slumped to a shock 33-28 loss at Bayonne on Saturday as the Basque side avenged their 59-0 rout earlier this season.
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 a game switched to San Sebastian in Spain.
Despite the defeat, 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 could snatch top spot from Toulon if they beat Biarritz in Saturday's late match.
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.
"I am really satisfied, it's huge what this group of players has achieved. I am happy for them," said Racing coach Gonzalo Quesada.
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.
Mont-de-Marsan have lost 20 of their 22 league matches this season.