Leinster surrendered their unbeaten record in this season's Heineken Cup as Northampton Saints won a low-scoring but absorbing contest at the Aviva Stadium.
Tries from George North and Jamie Elliott bookended this fifth round clash, as Jim Mallinder's men recovered resoundingly from last week's 40-point hammering from the Irish province.
Leinster had the opportunity to move 10 points clear at the top of Pool One, drawing a 47,370-strong crowd to the home of Irish rugby.
But Northampton - bottling the hurt caused by their collapse at Franklin's Gardens - landed an early blow with North's sixth-minute try and should have been further in front than 7-3 at the break.
North, Samu Manoa and Tom Wood helped the territorially-dominant Saints boss the physical stakes, turning the tables on the Irish province who were error-strewn during the first 40 minutes.
An Ian Madigan penalty in the 11th minute put four points between the sides, with the visitors' scrum- half Lee Dickson held up over the try-line and his half-back partner Stephen Myler missing a late shot at the posts.
Leinster resumed on the front foot with Madigan reducing the arrears to a single point.
Myler and Madigan exchanged penalties in the final quarter before Northampton held their nerve superbly at the finish, as Kahn Fotuali'I dropped a goal and winger Elliott ran in an intercept score in injury time.
The result, which brought back memories of Northampton's round 5 win at Ravenhill last December,
keeps Saints in the pool reckoning and leaves Leinster licking their wounds approaching Christmas - something they had to do 12 months ago when beaten at home by Clermont Auvergne.
Both sides were forced into late switches with a dead leg ruling out former European Player of the Year Sean O'Brien. Shane Jennings joined the Leinster back row, while Dom Waldouck's absence in the Northampton back-line saw North pair up with Luther Burrell in the centre for the first time.
Northampton had the first glimpse of the whitewash, springing teenage winger Tom Collins free from a solid scrum and Rob Kearney cynically bringing him down as they chased a kick.
Saints went for the line-out option and after second row Lawes was held up, Dickson and Burrell combined to release North for a close range try converted by Myler.
Luke Fitzgerald wriggled into space to set up field position for Leinster, with Madigan getting them off the mark in the 11th minute. But dropped balls and misjudged kicks made for a sloppy start from the hosts.
Leinster were forced on the defensive again as Wood wielded influence at the breakdown, before Brian O'Driscoll was invited forward by a short Jack McGrath pass.
Saints continued to force the issue though, with North intercepting a Fitzgerald pass and a dogged drive from the forwards almost led to a pushover try for busy scrum-half Dickson - Mike Ross held him up.
Rob Kearney, who was particularly busy in defence, tried to lift Matt O'Connor's men with two terrific touch-finding kicks - but the Northampton line-out was well-oiled with Christian Day in stellar form.
Myler misjudged a last-minute kick from the left hand side, failing to reward an advancing scrum, and Saints had to weather a Leinster storm in the opening minutes of the second half.
McGrath and Fitzgerald had Leinster within a metre of the try-line, Northampton's defence holding firm until Madigan put the minimum between the sides.
Saints went in search of a second try, winning consecutive penalties inside the 22 as they continued to back their line-out. An important tackle by Jennings on Manoa forced a relieving knock on.
A weaving run from the electric Fitzgerald brought Leinster downfield and as play continued to break up, North's deft hands released Ken Pisi for a raid into the 22 and Elliott was just unable to collect the final pass.
Three points from the boot of Myler were a decent reward for Saints' unrelenting endeavour.
Leinster then bludgeoned into scoring range with captain Jamie Heaslip and Gordon D'Arcy carrying well.
The province's hard graft earned a central penalty which Madigan dispatched with eight nerve-jangling minutes left.
Northampton displayed their resilience again when replacement scrum-half Fotuali'i clipped home his drop-goal, making up for a wayward 52-metre penalty from Meyler.
Leinster piled forward in the dying minutes, their forwards scrambling to make the try-line.
Northampton's defence almost gave way amid intense pressure, but a Heaslip fumble in the 22 allowed Elliott to gather and race away for the clinching score.