It was only a small yellow card, but it might have ended Leinster's hopes of retaining their Celtic League title.
Leinster prop Niall Treston was sin-binned for a professional foul late in the first half against Cardiff at the Arms Park tonight, and a 7-6 Leinster lead was a 7-24 deficit by the time he got back on the field.
Leinster, missing 11 first-choice players to Ireland's national squad, have lost three matches on the trot, and face a huge task to get back into title contention.
Cardiff's win was only Wales' fourth Celtic League victory over Irish opposition in 30 attempts.
Only the sniping of Leinster scrum-half Brian O'Meara and the power play of No 8 Des Dillon gave visiting fans anything to cheer about.
For the home fans, block-busting Canadian No 8 Dan Baugh was again the hero with some surging runs, while Harris' six penalty goals equalled his match-winning effort against Borders last weekend.
Scottish referee Iain Ramage was so keen to be part of the game, he was knocked over twice. Given his penchant for whistling, most fans were hoping he'd go down a third time.
After a clumsy start, Cardiff hit their straps in the final minutes of the first half to go into the break 9-7 ahead, and began the second with a stunning burst of action against the 14 Irishmen.
Fly-half Iestyn Harris belted a 50m clearance kick, Cardiff stole Leinster's lineout ball and quick ball gave winger Anthony Sullivan the time and space to score in the corner.
Harris added the extras from the touchline and his fourth penalty goal a minute later to finally give the home crowd something to cheer about this season.
More was to come - a great burst from scrum-half Ryan Powell set up quick ball for captain Martyn Williams to score in the same spot as Sullivan.
Leinster hit back when inside centre David Quinlan slipped some weak tackles to put lock Aidan Kearney over the try-line.
Andy Dunne's conversion cut the margin to 10 points, and his penalty goal moments later narrowed it to seven with 14 minutes remaining.
Arms Park fans braced themselves for another final-minute collapse from their side, but Harris' fifth penalty goal on the 70-minute mark gave his team some breathing space, and his sixth in the last minute was icing on the cake.
Early signs were bad for Leinster, with the home side making 30 easy metres with a rolling maul the Irishmen simply couldn't stop.
The ball went wide to Cardiff new boy Donavan van Vuuren, who was bundled into touch 10 metres out.
Harris opened the game's scoring with a penalty goal in the 11th minute after a Leinster lineout infringement.
Harris doubled the score in the 19th minute after some excellent buildup work from winger Sullivan and Harris himself put the home side within range. Leinster obliged with the infringement, and shots from in front are bread-and-butter for the former league star.
O'Meara missed a long-range penalty-goal attempt, but the visitors continued to press the home try-line and were rewarded when rangy No 8 Des Dillon wriggled out of the Cardiff defence to ground the ball over the line. O'Meara's conversion gave his side the lead, 7-6.
The little scrum-half was proving a thorn in Cardiff's side, and his excellent break on the half-hour mark should have resulted in a five-pointer, but flanker Aidan McCullen's hands let him down.
Harris failed to capitalise, missing a sitter in the 38th minute, but made amends on the stroke of half time when Treston was sin-binned.
Cardiff had the chance to go into the break with a bigger lead after a stirring run from inside their 22m involving most of the team, but van Vuuren was penalised for holding the ball in the tackle, with the try-line and half his team begging for it.
Tries - A Sullivan, M Williams;
Pens - I Harris 6;
Cons - I Harris.
Tries - D Dillon, A Kearney;
Pens - A Dunne;
Cons - B O'Meara, A Dunne.
Halftime: Cardiff 9-7.
Yellow card: N Treston (Leinster 39).
D van Vuuren; N Walne, J Robinson, P Muller, A Sullivan; I Harris, R Powell; T Payne, G Woods, G Powell (K Fourie 65), H Senekal, A Jones, R Appleyard, D Baugh (D McShane 65), M Williams (capt).
C Warner; K Lewis, S Horgan, D Quinlan, G D'Arcy; A Dunne, B
O'Meara (B Willis 61); E Byrne (P Coyle 61), P Smyth (capt), N Treston, B
Gissing (N Breslin 41), A Kearney, A McCullen, D Dillon, L Toland (S Jennings 61).
Carmarthen Quins are holding an exhibition to commemorate club players who fought in World War I. All welcome to the clubhouse on Friday afternoon to learn more about local history, or share any family anecdotes or artifacts.
Defence coach Shaun Edwards believes Saturday's clash against England - the 'form team in world rugby' - poses a huge challenge for his side but he is confident, with Wales beginning to show the defensive qualities displayed in the last two World Cups,