Saracens missed out on the chance to book a Champions Cup round of 16 home tie as Castres produced a stunning victory in London.
Tom Willis surged over for two tries as Saracens dispatched second-placed Bristol Bears in a 35-26 victory in the Gallagher Premiership on Saturday.
Bath inflicted a record Gallagher Premiership defeat on 14-man Saracens as they powered to a 68-10 victory at the Recreation Ground.
Saracens climbed up to third place after a comfortable victory over Northampton in the Gallagher Premiership.
A round-up of Saturday's Investec Champions Cup action, where Northampton and Saracens made winning starts but Exeter were beaten by Sharks in South Africa.
Six-try Bath powered their way to a bonus-point victory over Sale Sharks to round off the first block of Gallagher Premiership matches in style.
Alex Lozowski booted a penalty with the game's final kick as Saracens claimed a thrilling 37-35 Gallagher Premiership victory over Bristol at Ashton Gate.
Harlequins ended Saracens' 100 per cent record in this season's Gallagher Premiership by defeating their London rivals 17-10.