Leinster have tasted defeat in Europe for the first time in 18 months.
The reigning champions fell to a first Champions Cup loss in eleven games on Sunday afternoon, coming up a single point short of Toulouse, and they will know all too well that they were complicit in their own downfall.
It was Clermont Auvergne who had their number in a tournament semi-final back in April of 2017 and, as was the case that day in Lyon, it was a terrible opening spell that ultimately cost the province dearly.
Leo Cullen's side improved immeasurably after that but they never managed to get a handle on this game against the only other club to have won this competition four times. The losing bonus point will be little compensation for now.
Heralded from all corners of the European game after their demolition of Wasps last week, Leinster were every bit as bad here in the opening quarter as they had been good in that 52-3 thrashing of the English premiership side.
Error after error stacked up and, with Toulouse clearly of a mood to do some damage, the result was a torrent of pressure that fashioned a 14-0 lead for the home side through the opening 21 minutes at a sunkissed and atmospheric stadium.
Thomas Ramos got the crowd going with a pair of penalties as Leinster fumbled and foostered for some rhythm. Sean Cronin had a lineout stolen, Robbie Henshaw got stripped, Rhys Ruddock turned over. Joe Tomane got done for a forward pass and Jordan Larmour ran down a blind alley.
All very unLeinster-like.
The first Toulouse try actually stemmed from Leinster's first bit of fluid play. Luke McGrath peeled off a good scrum to make a heap of metres but then a Cronin offload was intercepted by Alban Placines and taken on through open prairie by the hooker Julien Marchand.
Nearly 50 metres had been gained in the blink of an eye and Leinster were still in scramble mode by the time it was recycled and worked left to Maxime Medard. The veteran wing had all the time and space in the world to outfox an exposed Joe Tomane and claim the five points.
Another Ramos penalty made it 14-0 and it looked like being 21 and game all but over when Cheslin Kolbe intercepted a pass from Larmour – who had a poor first-half – only for referee Wayne Barnes to call play back for an earlier offside.
Jonathan Sexton claimed the first of Leinster's penalties from that. Another was added five minutes later before Sean O'Brien, an 18th-minute replacement for Josh van der Flier, stormed over for a try off the shoulder of Jack Conan who had made a superb line break.
Sexton's extra left it at 14-13 with the break approaching but Toulouse struck again off a scrum near the Leinster 22 with Selevasio Tolofua standing O'Brien up and releasing Sofiane Guitoune who jinked past Larmour for a score that would be worth seven.
Trailing by eight at the break, Leinster couldn't allow the hosts any more wiggle room and they went about their business in a much more clinical manner after the break with a pair of tries that put them half-a-dozen points in front and within distance of the win and a try bonus point.
Both scores were built on bloody-mindedness and old-fashioned physicality. The hard yards put in by the forwards – all of them – was astonishing on what was a heavy surface and James Ryan claimed their second five-pointer with a short burst over the line.
Their third had a tad more artistry to it, Sexton feeding Cronin with a reverse pass after the grunts has softened up the line with another passage of pain and the galloping hooker dotted down under the posts for another easy conversion.
Cue the first real dip in a game of lurching emotions and momentum but one which ended 15 minutes later when, with Leinster probing the Toulouse 22, a Luke McGrath feed from a ruck was intercepted by Louis-Benoit Madaule.
The lumbering forward had open turf but knew his limitations and so followed a brilliantly intricate team move through the hands of Yoann Huget, Placines, Sebastien Bezy and, finally, Medard whose dive over the line was decorated by another Ramos two-pointer.
Toulouse led by a point with ten minutes to play. The drama was far from over but the scoring was. A day of days for Toulouse who were playing at home in the Champions Cup for the first time since failing to qualify for the tournament last time around.
T Ramos; C Kolbe, S Guitoune, R Ntamack, M Medard; Z Holmes, S Bezy; C Castets, J Marchand, C Faumuina; F Verhaeghe, J Tekori; R Elstadt, A Placines, S Tolofua.
Y Huget for Ntamack (23-); L Ghiraldini for Marchand, A Dupont for Holmes and M van Dyk for Faumuina (all 55); LB Madaule for Tekori (57); A Ainu'u for Castets (62).
J Larmour; J Tomane, G Ringrose, R Henshaw, J Lowe; J Sexton, LMcGrath; C Healy, S Cronin, T Furlong; D Toner, J Ryan; R Ruddock, J van der Flier,J Conan.
S O'Brien for van der Flier (18); JMcGrath for Healy (55); A Porter for Furlong and J Tracy for Cronin (both 56); S Fardy for Ruddock (66); N McCarthy for L McGrath (74).
W Barnes (Eng).