Racing 92 have their work cut to qualify for the European Champions Cup quarter-finals after suffering a shock defeat to Glasgow Warriors, while Ulster claimed an impressive win over Clermont Auvergne and Munster consigned Leicester Tigers to a record loss
Glasgow's hopes of qualifying for the knockout stages suffered a major blow when they were beaten at Stade Yves du Manoir last season, but the Pro12 side gained sweet revenge with a stunning 23-14 victory in Pool 1 on Saturday.
Gregor Townsend's men produced a heroic display against the Top 14 champions and came out on top to sit a point behind leaders Munster courtesy of tries from Alex Dunbar and Ali Price, as well as 13 points from the boot of Finn Russell.
Juan Imhoff put last year's runners-ups Racing in front with an early try and Dan Carter waltzed through for a second Racing score, which he converted.
Yet Glasgow were not to be denied a third win in France which leaves the hosts bottom without a single point after two games on a memorable day for some of the top clubs in the Pro12.
Ulster defied the odds to upset Top 14 leaders 39-32 at Kingspan Stadium, where Luke Marshall crossed twice, Charles Piutau touched down and Iain Henderson registered a five-pointer.
There was also a try for Paddy Jackson, who amassed 19 points in Belfast to leave Ulster just three points behind Pool 5 leaders Clermont.
Munster are sitting pretty at the summit of Pool 1 following a humiliating 38-0 trouncing of Leicester Tigers, who slumped to their biggest ever European defeat at Thomond Park.
Jaco Taute scored two of Munster's four tries, while while the returning Manu Tuilagi, Ed Slater and George Worth were all yellow carded on a miserable day for the woeful Premiership side.
Maro Itoje was man of the match as champions Saracens made it three wins out of three in Pool 3 by running in six tries in a 50-3 mauling of Sale Sharks, stretching their winning run in Europe to 12 matches.
Toulouse won the only Pool 2 contest of the day to go second, claiming a 36-6 in a contest which was called off due to fog after 63 minutes but the result stood.