Rarely has a victory looked so much like a loss.
Leinster owned 58% of the possession and the territory, made more carries (114-103), kicked more (32-30) and recycled more rucks and mauls (102-65).
With all this in their favour, Harlequins easily made more metres (377-259), beat more defenders (13-11), made more line breaks (4-3) and hoovered the offloads (8-0).
In short, Conor O'Shea's men played the better, more attractive and more effective rugby.
And still. The way Jamie Heaslip clenched his fist at the final whistle suggested otherwise.
For the man who is all about winning might have realised this was one that could so easily have got away from Leinster completely.
The Blues were standing on the brink of European oblivion as they watched an eleven-point lead turn into a two-point deficit through the combination of Harlequins initiative and their own inaccurate play.
There was no real basis to assert they deserved it. They scrapped away. They stayed in it. And they had real, torch-lighting impact from Eoin Reddan off the bench.
Twice the scrum-half, so off-colour a week earlier, got in behind Quins to ask questions the Englishmen couldn't really answer and Ian Madigan clipped the match-defining points for 14-13 in the 70th minute.
Coach Matt O'Connor did not look for an easy way out: "We didn't get to the level we wanted to be at," he said.
"It was about keeping ourselves alive in the group and we've done that. We can improve. We've got lots of growth there."
Mike Ross is known as a problem-solving tight-head with the nous to adapt to what is happening, literally, right under his nose.
For the second time in six days Harlequins ruled the resets.
"There were errors and there were opportunities that we didn't take and there were set-piece issues," continued O'Connor.
Harlequins dominated there again through Joe Marler, Dave Ward and Will Collier and also forced Sean Cronin's hand at the lineout where George Robson was a formidable presence.
Leinster were beaten at the set-piece and beaten up physically. Nowhere was it more etched than in the face of Jack McGrath as he trundled off in the 76th minute.
In what is a specialist area, you don't have to have propped at any level to see Harlequins had the shove on Leinster at scrum time, however they managed to do it.
It all looked so much better at the end of the first-half. Leinster built steadily from Madigan's six points in the 25th and 37th minutes, sandwiching Isaac Boss's benefit from Heaslip's cool profit out of a two-on-one with Aseli Tikoirotuma.
Certainly, Tim Swiel did little or nothing to swell Harlequins' bellies for the fight. He fed their appetite with empty promises from two wasted penalties, the second a 'gimme' at this level.
The four tries looked an outside possibility at the break. As it turned out, it was Leinster who needed a break to win it.
Full-back Rob Kearney was lucky to avoid caution when he appeared to rip the ball from the hands of Marler on the ground which would have led to Mike Brown's try.
As it transpired, Brown was denied by the eyes-wide-closed view of The Match Official Frenchman Eric Gonthier.
It was justice denied, but also only justice delayed as Brown made his way over unopposed in the 56th minute.
They had the momentum. Even Swiel started to come on board the confidence train. He shot the lead points when Devin Toner tackled Jack Clifford without the ball in the 65th minute to have Leinster staring down the barrel of a gun.
It took the interventions of Reddan to take Leinster up a gear and for Madigan to squeeze through the winning points.