Ronan O'Gara kicked a controversial, late 50m penalty to hand defending champions Munster a 21-19 win over Leicester in a rain-swept European Cup tie on Sunday.
The Irish international kept his nerve to score his second penalty of the game to add to a conversion and a drop goal.
But Leicester, who had battled back from a 15-6 half-time deficit to lead 19-18 with the seconds ticking away at a wild and wet Welford Road, believed that O'Gara had knocked the ball on in the move which led to the penalty award.
Munster were on top in the first half through tries from Donnacha O'Callaghan and David Wallace.
Leicester brought on Andy Goode for Paul Burke at the break and he went on to kick two penalties and a conversion of a penalty try which was awarded for constant Munster infringements.
The Tigers, champions in 2001 and 2002, then led for the first time through a Goode penalty before O'Gara settled matters two minutes from time.
O'Gara, who criticized the standards of English rugby in midweek, admitted he had knocked the ball on.
"I lost control of the ball before the penalty was given. The referee gave it for a high tackle and then there was some back-chat," he said.
Leicester coach Pat Howard was philosophical in defeat.
"Rugby is a game of very small margins, and Ronan O'Gara stood up and kicked the penalty brilliantly," Howard said.
"It was a very close contest and we probably had a couple more chances than Munster did, but they took their try-scoring opportunities. That's life," he said.
Wasps, the 2004 winners, saw off Castres 19-13 after edging ahead through a Paul Sackey try, and a conversion and three penalties from Jeremy Staunton.
They went off the boil after the break with just a late penalty from Mark van Gisbergen to show for their efforts.
Castres hit back with two penalties from Gordon Ross, who also converted a second-half try from Akventsi Giorgadze.
Biarritz, the runners-up last season, began their campaign with a 22-10 defeat of 2000 champions Northampton.
Fijian full-back Sireli Bobo scored two tries for Biarritz with Mohammed Dridi and hooker Benoit August adding the others while Northampton's chances were hit by the absence of star fly-half Carlos Spencer and winger Ben Cohen.
"We have got to be realistic. We made too many mistakes in the first half," Northampton coach Paul Grayson said.
"They took their chances and put us on the back foot, so we gave ourselves a mountain to climb," he added.