Leinster's six-game Pro 12 winning streak came to an abrupt end as Scarlets produced a stunning display to inflict a first defeat on the leaders since September.
Five tries secured a 38-29 bonus-point win for Scarlets and lifted them into the top four, while extending their own Pro 12 unbeaten run to six.
Scarlets did most of the damage in the opening 40 with tries from Johnny McNicholl and Rhys Patchell, with Will Boyde going over early in the second half after Barry Daly had seen red for Leinster.
The Irish side battled valiantly to earn a losing bonus but one solitary point leaves them in danger of being usurped by Munster on Saturday.
Cardiff Blues' slump continues as they were beaten 18-7 by Connacht, a fourth loss in the last five for the Welsh side.
John Cooney proved the difference with 13 of Connacht's points as their revival continues, this their third win of the campaign to reignite their play-off dreams.
A 22-5 victory over Glasgow Warriors got Ospreys back to winning ways after their defeat to Munster last time out, but for the Warriors it leaves their top-fourth berth under threat.
Tries from Dan Baker, Ashley Beck and Tom O'Flaherty saw Ospreys score 17 points in the second half - in which Glasgow were reduced to 14 - to move above Munster into second and to within a point of Leinster at the top.
Friday's final game fell victim to the weather, however, as a frozen pitch was deemed unsafe by the referee ahead of Ulster's match with Zebre.