Defending champions Leinster face an uphill task to reach the Heineken Cup quarter-finals after their second successive defeat against Clermont.
Clermont, who won 15-12 in the reverse fixture, held a commanding 25-9 lead after Morgan Parra kicked 20 points and Wesley Fofana scored a first-half try.
But the holders claimed a vital losing bonus point after late tries from Shane Jennings and Fergus McFadden.
Clermont hold an eight-point advantage over Leinster at the top of Pool Five.
Joe Schmidt's men, who beat the French side at the semi-final stage of last season's competition, know overhauling the leaders to qualify outright for the quarter-finals looks a slim possibility.
However, maximum points from their final two matches against Exeter and Scarlets could see the Irish side qualify as one of the group stage's two best runners-up.
Clermont produced an impressive display of physical power in Dublin, where Leinster had not lost since October 2009, to outline their credentials as one of the tournament favourites.
The game was finely poised in the opening quarter as both defences held firm, before the momentum swung in the French side's favour with the game's first try after 34 minutes.
Sitiveni Sivivatu exploited a small gap in the Irish side's defence before brilliantly off-loading to Napolioni Nalaga, who was held up by desperate home defending.
However, the hosts could not regroup quickly enough as the ball was recycled and Fofana, who famously lost control of the ball as he thought he had scored the match-winning try in last season's semi-final, powered over on the angle.
It was a sucker-punch for Leinster, who would not have been too disheartened by limiting the visitors to just a 9-6 half-time advantage.
But they made a bad start to the second period when Mike Ross lost his footing and collapsed the scrum to leave Parra with another kickable opportunity, which he took.
Jonny Sexton's penalty reduced the deficit to 10 points shortly after, but another attempt from the halfway line dropped agonisingly wide of the posts.
However, any sense of optimism from the home crowd soon evaporated when their side were reduced to 14 men.
Sean O'Brien clattered Sivivatu in a cynical off-the-ball tackle to halt a quick Clermont counter instigated by Parra's sharp-thinking tap.
France international Parra booted over the resulting penalty and the numerical advantage allowed Clermont to crank up the pressure deep in Leinster territory.
Sivivatu felt he had barged over the line seconds before O'Brien's return, but video referee Geoff Warren decided several replays proved inconclusive and the try was ruled out.
O'Brien returned to the field as Leinster survived their numerical disadvantage without further loss.
Suddenly the home crowd found their voice as they tried to spur their side towards a dramatic bonus point which could prove crucial after the final two pool games next month.
A quick line-out and strong drive allowed Jennings to surge through the French defence and Sexton converted as Leinster reduced the deficit to 25-16 with 13 minutes remaining.
Parra, who looked assured every time he was handed a kicking opportunity, added another penalty as closing the gap to seven points or less looked beyond Leinster.
But one last burst in the final play of the match allowed McFadden to score and give the 48,000 crowd, an Irish record for a Heineken Cup pool match, faint hope of a last-eight place.
Leinster head coach Joe Schmidt:
"Miracles happen in sport, I guess, but mathematical long shots very seldom come into being.
"I don't think we have the same depth as some of the big-spending French clubs. It's hard to take defeat two weeks in a row, with the amount of effort the players put in.
"The other challenge for champions is they always look pretty good when they are on top. It is how they come back from adversity that is the real challenge."
Leinster: Madigan, McFadden, D'Arcy, Goodman, Nacewa, J Sexton, Reddan, Van der Merwe, Strauss, Ross, Cullen, Browne, O'Brien, Jennings, J Heaslip.
Replacements: D Kearney for Nacewa (59), Boss for Reddan (66), Healy for van der Merwe (47), Cronin for Strauss (12), Bent for Ross (52), Toner for Browne (41), Murphy for Jennings (67). Not Used: Conway.
Sin Bin: O'Brien (51).
Clermont Auvergne: Byrne, Sivivatu, Rougerie, Fofana, Nalaga, James, Parra, Chaume, Kayser, Zirakashvili, Cudmore, Hines, Bonnaire, Bardy, Chouly.
Replacements: King for Byrne (37), Debaty for Chaume (66), Paulo for Kayser (70), Kotze for Zirakashvili (70), Jacquet for Hines (75), Lapandry for Bonnaire (68). Not Used: Radoslavjevic, Skrela.
Referee: Wayne Barnes (RFU)