South Africa's World Cup fly-half Manie Libbok broke French hearts again on Saturday with a last-minute conversion for the Stormers that sent holders La Rochelle spinning to a 21-20 defeat in Cape Town, while last season's beaten finalists Leinster came from behind to win against Sale.
Three-time champions Saracens, meanwhile, got their first win in a 55-36 romp at home to Irish province Connacht, while Bordeaux-Begles overpowered Bristol 36-17.
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La Rochelle, who were beaten at home last week by Leinster in a repeat of the last two finals, began strongly with former All Black scrum-half Tawera Kerr-Barlow crossing in the first half for a 13-6 lead.
Yoan Tanga then appeared to put the French out of sight with a third try.
But Ben Loader pulled a try back for the South African franchise before a double twist firmly shifted momentum their way.
La Rochelle fly-half Antoine Hastoy had a try ruled out for an infringement in the build-up by Joel Sclavi, who was then given a yellow card.
Using the man advantage, Andre-Hugo Venter crashed over in the last minute with Libbok, so often criticised for his goalkicking during the World Cup, sending the ball between the posts to claim the win.
"We're disappointed, we came here to win," said Wallabies lock Will Skelton.
"Against a team like the Stormers, you can lead the whole match but if you give away penalties, it's going to cost you in the end. That's what happened."
With games to come in January against English sides Leicester and Sale, there is a real danger that the two-time champions could be eliminated before the knockout stage.
Leinster, meanwhile, trailed 13-3 to Sale just before half-time in Dublin but turned the game around with a thrilling performance in the second half.
Josh van der Flier dotted down just before the break and that was followed by further touchdowns for Jamison Gibson-Park, Robbie Henshaw, Ryan Baird and Cian Healy.
"We were a bit sluggish and not as good as we'd have liked to be at the start," Van der Flier said after the game.
"We brought good momentum and physicality in the second half. Disappointing to coincide two tries in the last few minutes but overall (we're) very happy with that.
Leinster's 37-27 win earned them a bonus point and they top Pool 4, with a four-point lead over Leicester in second.
Saracens got their quest for a fourth European title back on track following a defeat against the Bulls in Pretoria last weekend with a bonus-point triumph over Connacht in Pool 1.
It was not all plain sailing, with the Irish province nine points ahead coming up to half-time but two tries from England hooker Jamie George gave Saracens a 21-17 lead at the break.
That came in the middle of an extraordinary passage when Saracens scored 35 unanswered points between the 33rd and 61st minutes.
Centre Olly Hartley scored twice in the second half as the English club eased clear but Connacht's five tries, including one for Ireland centre Bundee Aki, saw them collect a losing bonus point.
Connacht were hammered 41-5 at home last week by Bordeaux who continued their flying start to the campaign with a second bonus-point win over Bristol, France wings Damian Penaud and Louis Bielle-Biarrey were among their five try-scorers.
Lyon were another side who needed a late finish to secure a win, Thibaut Regard's 72nd-minute try edging them to a 29-28 home win over the Bulls.
Bath, the 1998 champions, moved to the top of Pool 2 with 10 points out of 10 after a 10-try thriller at Cardiff.
The home side pushed them every inch of the way, and led until South African forward Jaco Coetzee touched down twice during the final quarter as Bath ran in six tries.
Cardiff collected two losing bonus points for their efforts with Wales wing Josh Adams scoring twice before going off injured in a 39-32 defeat.
Ulster bounced back from defeat to Bath last week with a 31-15 victory over French Top 14 leaders Racing 92, whose indiscipline cost them dear as they slipped to a second successive defeat.
Flanker Nick Timoney scored in each half for Ulster with Stuart McCloskey and Matty Rea also crossing for the hosts.
Round two concludes with three matches on Sunday including Harlequins at home to French champions Toulouse.
