Wasps crashed to a shock 32-31 loss to Edinburgh in their European Cup opener on Sunday but Welsh side Ospreys held on for a stunning 13-8 win over last season's beaten finalists Stade Francais.
To compound a bad day for French clubs, Saracens posted a 22-10 home win over Biarritz, who made it to last year's semifinals.
At Murrayfield, Simon Web-ster came off the bench to score the decisive try as the Scottish club achieved one of the biggest upsets in the tournament's history.
Paul Sackey's late try for Wasps looked to have sealed victory for last year's champions but Scotland international Webster then made the most of a handling error by Sackey to race over in the first minute of injury time.
Edinburgh captain and Scotland Test regular Chris Paterson missed his conversion from under the posts but the Gunners held out for seven nerve-wracking minutes of stoppage time to stun Law-rence Dallaglio's Premiership giants.
"It was a fantastic display and a great win," said Paterson, who was suffering from a heavy cold and missed three penalties and two conversions.
In the Ospreys' impressive new Liberty Stadium, which they share with Swansea City football club, Stade were unable to break down a teak-tough home defense led admirably by Welsh internationals, center Sonny Parker and man-of-the-match flanker Jonathon Thomas.
Ospreys, missing a clutch of key players including Gavin Henson and Ryan Jones, scored their solitary try through Parker, who latched on to a sweet inside pass from outside half Shaun Connor who dummied through the Stade line.
Connor was successful with the conversion and also got a late first-half penalty in response to an earlier one by Stade fullback Juan Hernandez.
Stade winger Lucas Borges claimed a try back with 14 minutes to go but a late penalty by Matthew Jones and some gritty defense ensured a memorable victory for the Welsh side.
Stade coach Fabien Gathie was in a gloomy mood after the match.
"We will do our best to play well next weekend [against Leicester], but it is going to be hard. When you lose the first match in this competition, it is always difficult to qualify," he said.
In the third match of the day, Saracens marked their return to the European Cup after a five-year absence with an impressive 22-10 win over Biarritz at Vicarage Road.
Man-of-the-match Glen Jackson kicked 14 points and set up Ben Johnstone for Saracens's first-half try with a well-timed pass.
Almost an hour had elapsed before Biarritz got on the scoreboard thanks to a penalty from France's Dimitri Yachvili.
He later converted a Julien Peyrelongue try but Saracens had the game wrapped up.
Such was the home side's dominance, captain and loose forward Hugh Vyvyan even landed a 40m drop-goal.
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