Joshua Kenny scored two second half tries as Leinster overturned a half-time deficit to claim a bonus point win away to Dragons at Rodney Parade.
It was another ragged performance from the champions, who were shown three yellow cards and trailed at half-time, with Leo Cullen afterwards lamenting bouts of poor discipline and squandered chances.
Cullen's side entered the game with just two wins from their opening five games but got off to a quick start, with Alex Soroka touching down after four minutes after beasting past a couple of tacklers close to the right touchline.
However, the conversion was missed and Leinster were reduced to 14 after back-rower Diarmuid Mangan was sin-binned on the 29 minute mark.
The Welsh side, who'd been piling on the pressure at that stage, got over within a couple of minutes, Matthew Screech powering over from close range under the posts.
In an uncomfortable echo of Ireland's loss to South Africa, Leinster were briefly reduced to 13 men as Scott Penny was sin-binned after 36 minutes, though the visitors managed to avoid shipping another points in this spell.
Behind 7-5 at the break, Leinster did spark to life early in the second, Kenny scoring his third try in as many games after quick hands from Harry Byrne and Ciaran Frawley put him over in the right hand corner.
The Greystones RFC graduate scored his second just on the hour mark to put some daylight between the sides, Kenny bouncing off a defender to touch down in the corner after a clever over-the-top floated pass from RG Snyman.
Leading 17-10, Leinster were living on their nerves near the end and got lucky on 75 minutes, when Penny controversially escaped a second yellow card after appearing not to wrap when making a tackle close to the try line, catching the ball carrier high with his shoulder.
After consulting, the referee decided that he was attempting to wrap his arms and that there was no foul play involved. It was a call that left Dragons head coach Filo Tiatia frustrated, the New Zealander saying afterwards that it was "clearly shoulder contact to the head."
It was the big South African lock who assured Leinster of the win and secured the bonus point with the fourth try two minutes from time.
The second rower capitalised on Huw Anderson dropping Luke McGrath's garryowen under pressure. Snyman caught the bouncing ball, handed off a couple of tackles and galloped down the wing - Noel Mannion-style - to score in the corner.
McGrath became the third Leinster player to go in the bin for failing to retreat 10 metres at the death but it was immaterial at that point.
Afterwards, Cullen admitted he was "hugely frustrated" with the performance though he accepted the result was a "great outcome" heading into their Champions Cup opener at home to Harlequins next weekend.
