Jacob Stockdale's try-scoring blunder will haunt Ulster and the winger after defending champions Leinster came from behind to win a gruelling all-Irish Heineken Champions Cup quarter-final 21-18 at a packed Aviva Stadium.
The joint-leading try scorer from the pool stages with six, Stockdale crucially lost control of the ball past the try-line early in the second half, and despite Luke Marshall's subsequent try bringing them level, the Ulstermen fell short thanks to Ross Byrne's 71st-minute penalty a dominant final 41 phases of possession rugby from the hosts.
A surprise result looked a possibility when Kieran Treadwell and Byrne, who deputised at fly-half for the injured Jonathan Sexton, swapped tries inside the opening 10 minutes.
Despite losing captain Rory Best to an early ankle injury, John Cooney's eight points from the tee had Dan McFarland's underdogs 13-11 ahead at half-time.
It was a tale of two wingers when Stockdale agonisingly dropped the ball from his grasp, and a subsequent Leinster attack sent Adam Byrne over for a momentum-shifting 53rd-minute score.
They had to show huge resilience in the end, coming through the setback of Dan Leavy's horrific knee injury and Marshall's levelling five-pointer to settle the issue through Byrne's reliable right boot.
Playing European knockout rugby for the first time since 2014, Ulster tore out of the blocks.
They were led by the talismanic Best, who had an early turnover and then charged down Garry Ringrose's kick close to the Leinster posts, the loose ball gleefully touched down by lock Treadwell for a converted try with just five minutes gone.
Ringrose redeemed himself with some clever footwork as wind-backed Leinster ominously advanced through 25 pulsating phases, at the end of which Sexton's deputy Byrne crossed out wide despite Robert Baloucoune's tackle.
Full-back Jordan Larmour's evasive running was a feature of the build-up, although Byrne's missed conversion kept the Ulstermen in front.
The 23-year-old Dubliner missed a central 41-metre penalty in the 18th minute, soon after Best's worrying injury for both province and country.
Cooney rewarded a fine aerial take by Stockdale and a meaty Marcell Coetzee carry with three points, while Leinster fluffed their lines in response, Sean Cronin bursting up towards the 22 but then overthrowing the resulting lineout.
Leinster's accuracy returned on the half-hour mark when Byrne split the posts and then a scrum penalty soon allowed the fly-half to briefly nudge the hosts ahead for the first time.
He almost teed up a try for Dave Kearney before that, connecting with the winger from a cross-field kick but Baloucoune saved Ulster's bacon. Michael Lowry and Billy Burns added zip to a late Ulster attack which Cooney topped off with three more points.
Leinster had two big let-offs early on the resumption, the ball going dead after Ringrose was charged down again, and then Stockdale did all the hard work in brilliantly surging clear down the left touchline but he lost control of the ball as he stooped to score under pressure from Kearney. It was an individual error which Stockdale and Ulster were left to rue.
Leo Cullen's men lifted the tempo decisively, Cronin threatening more in the loose albeit that Cooney foiled a promising first wave.
Seizing his chance, Jack Conan then cut straight through from a ruck to offload for the trailing Adam Byrne to finish off from the right wing. The extras were added by Ross Byrne, establishing a five-point cushion with both benches increasingly busy in this bruising repeat of the 2012 final.
The luckless Leavy had to be stretched off just as Ulster turned down a kickable penalty. Their directness was rewarded as they went through 11 phases off a lineout and an excellent delayed pass from Stuart McCloskey opened up an avenue for Burns to put Marshall crashing over to the right of the posts. The pressure got to the usually cool Cooney who curled his conversion wide at 18-all.
Both sides were really emptying the tank down the final stretch, the Leinster pack using their maul to eke out a penalty on the right wing and Byrne, who had been struggling with cramp, stood up to the task to slot over what proved to be the match-winning kick.
Showing their experience, the title holders owned the ball for the remainder, relying on their pick-and-go game to confirm their qualification for an Easter weekend semi-final against either Racing 92 or Toulouse.
