Cullen will be furious but could prove to be a timely reminder
After talking all about Leinster's incredible self-belief and confidence last week, Munster managed to grab all the bragging rights with their overdue home win in Thomond Park on Saturday.
This game was probably always going to be edgy given Munster's desire to get their season back on track, and add to that the fact that over the past few years Leinster has predominated dominated the local derby.
It was a feisty first half that saw Leinster suffer two yellow cards to Irish props Cian Healy (celebrating 200 matches for Leisnter) and Tadgh Furlong, then on 30 minutes a red card shown by an overworked referee Frank Murphy to aggressive winger James Lowe when he illegally took Andrew Conway out in the air.
After that Leinster were always going to find the win hard.
Leinster started well, with their backline proving sparkling enough but just not getting points on the board they may have deserved but after that the succession of penalties, and yellow cards meant they were chasing a game far earlier than their coach Leo Cullen would have anticipated.
Munster played the smarter game knowing that keeping it tight as Leinster were chopping and changing their team as a result of the cards.
Keeping the play tight was obviously an area where they fancied their chances, scoring their first try after a well-driven lineout maul.
Ultimately it proved to be the undoing of Leinster over the 80 minutes.
In truth Leinster probably felt more alive when the play became a little more disjointed during the second-half, that was before Keith Earls put the game beyond Leinster with an intercept try.
The ole cliché of the side that wanted it the most was victorious rang true this time, and that was Munster.
It was a game that they wanted and needed, especially as they look forward to a tough run in for both competitions.
The repeated Leinster ill-discipline will be one of the discussion points for Leo Cullen to address this week. He will be annoyed about that as it simply gives other teams a clue on how to upset his side in upcoming games.
Cullen knows that his players must stay focused at all times.
Not that it's the end of the world given Leinster outstanding record this year, and maybe the timely reminder that they must work hard for their wins.
Expect Leinster to put things right before the next match against an improving Ulster outfit next Saturday in the RDS (5.15) with Toulouse seven days later in the Champions Cup.