Scarlets took command of their Heineken Cup group with a rollercoaster 27-25 triumph over London Irish sealed by Stephen Jones' last-gasp penalty.
Trailing 21-18 with 15 minutes to go, prolific Samoan winger Sailosi Tagicakibau ran in his seventh try of the season to give the Exiles sight of victory.
But with little over a minute remaining on the clock, substitute Peter Richards conceded a penalty in front of the posts and Jones landed the decisive kick.
Wales fly-half Jones was in commanding form, kicking magnificently throughout to finish with 17 points and showing composure to help Scarlets remain top of Pool Six.
Irish edged the try count three to two with Declan Danaher and Elvis Seveali'i also crossing, but victory could have gone either way in a compelling contest at the Madejski Stadium.