Biarritz picked up a morale-boosting 15-10 victory over Saracens at Parc des Sports Aguilera in a triumph that may be priceless in how Pool 5 ends.
It took a remarkable 51 minutes for the first points to be scored in the game, through the boot of visiting centre Owen Farrell's penalty attempt.
But that only served to awaken BO, who claimed tries in 56th and 65th minute courtesy of flanker Imanol Harinordoquy and wing Taku Ngwenya.
Saracens will, however, be delighted by their late converted try from replacement back Alex Goode, which saw them leave France with a bonus-point.
With the game scoreless at half-time, Farrell moved to the fly-half position due to Charlie Hodgson leaving the field with a leg problem. He took eleven minutes to make his mark too with three points that finally broke the deadlock.
However, Biarritz then clicked into gear with a fine counter-attack from deep inside their own territory leading to a cross-kick finding Harinordoquy.
Replacement Julien Peyrelongue missed the extras but was on-target soon after with a penalty that put Biarritz 8-3 up before USA star Ngwenya burst clear on the halfway line to go in under the posts. Peyrelongue converted to further extend the lead in France.
But Saracens were to have the final word when Goode crossed in the left corner and Farrell added the two points as the English left with a consolation point that keeps them top.
Tries: Harinordoquy, Ngwenya
Biarritz Olympique : 15 Dane Haylett-Petty, 14 Taku Ngwenya, 13 Benoît Baby, 12 Damien Traille, 11 Iain Balshaw, 10 Marcelo Bosch, 9 Luix Roussarie, 8 Raphael Lakafia, 7 Imanol Harinordoquy, 6 Benoit Guyot, 5 Erik Lund, 4 Jérôme Thion, 3 Sylvain Marconnet, 2 Benoit August, 1 Fabien Barcella.
Replacements: 16 Arnaud Heguy, 17 Yvan Watremez, 18 Francisco Gomez Kodela, 19 Pelu Taele, 20 Florian Faure, 21 Yann Lesgourgues, 22 Julien Peyrelongue, 23 Ilikena Bolakoro.
Saracens: 15 Chris Wyles, 14 David Strettle, 13 Owen Farrell, 12 Brad Barritt, 11 James Short, 10 Charlie Hodgson, 9 Ben Spencer, 8 Ernst Joubert, 7 Jacques Burger, 6 Kelly Brown, 5 Hayden Smith, 4 Steve Borthwick, 3 Matt Stevens, 2 John Smit, 1 Rhys Gill.
Replacements: 16 Jamie George, 17 Deon Carstens, 18 Carlos Nieto, 19 Mouritz Botha, 20 Andy Saull, 21 Luke Baldwin, 22 Adam Powell, 23 Alex Goode.
Referee: Nigel Owens