Scarlets were knocked off the top of the Pro12 table on Friday courtesy of a second agonisingly narrow defeat over the festive period.
Having gone down 27-26 to Ospreys on Boxing Day, in a game that saw Steven Shingler miss a late penalty, Wayne Pivac's men conceded a late try to lose 29-27 to Cardiff Blues on this occasion.
The Blues were five points behind heading into the closing minutes, but Alex Cuthbert burrowed over from close range following a fine break from Tom James, enabling Rhys Patchell - who will join Scarlets at the end of the season - to land a match-winning conversion.
Scarlets may rue their lack of discipline after losing three men to yellow cards.
Their slip-up enabled Leinster to take over at the top on points difference as a result of their 13-0 victory over Connacht, who remain a point off the pace in third.
In miserable conditions at the RDS Arena, it took an hour for Josh van der Flier to register the game's only try - the flanker burrowing over after a spell of home pressure.
Ospreys' excellent form continued as they battled to a 12-7 victory over Dragons at the Liberty Stadium - their fifth successive win.
Tries from Dan Evans and Hanno Dirksen put the hosts in control, but Ed Jackson's second-half score set up a tight finish.