No other side annoyed Sir Alex Ferguson more than Liverpool.
And nothing got under the skin of the former Manchester United boss quite like a defeat to his bitter Red rivals from down the East Lancs Road.
That's exactly what happened 16 years ago today, when Peter Crouch's headed goal knocked United out of the FA Cup at Anfield and helped Liverpool on their way to lifting the trophy in 2006.
In a furious contest that saw Liverpool win the war of attrition, Alan Smith suffered serious injury blocking a late John Arne Riise shot and birthday boy Gary Neville was pelted with a half-eaten burger by the Anfield crowd.
But Liverpool's triumph was to prove the turning point in what had previously been a cordial relationship between Ferguson and Rafael Benitez, who was just over 18 months into his Anfield reign and up until that point, had failed to beat United in any contest.
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Ferguson had previously been polite towards his opposite number, but as the ECHO's Chief LFC Writer Ian Doyle, then writing for the Daily Post put it, the "mask slipped" when Benitez out-thought the Scot to end their hopes of another major trophy.
And with Liverpool closing ground in the Premier League, Ferguson didn't react well to being one-upped by the Spaniard.
"Liverpool can be that way," Ferguson moaned after seeing his side outplayed for 90 minutes. . "They play for five minutes and win a game. That's the way they are."
He'd add: "You need an element of luck in the Cup and we didn't have it.
"We lost Rio Ferdinand after training on Friday then Mikael Silvestre got a knock.
"It has been 85 years since Liverpool beat us in the FA Cup and I was hoping it would be another 85 years, then I wouldn't be around to see it."
In his report, Ian Doyle continued: "Even by Ferguson's increasingly bizarre standards, it's a statement that will take some beating. The irascible Scot had been surprisingly generous in his praise of Benitez before the game - nothing to do with his previously unbeaten record against the Spaniard, of course - but the mask soon slipped once he had been comprehensively outsmarted by the Anfield manager.
"In doing so, Benitez scored his first victory over United at the fifth attempt and ended 85 years of FA Cup hurt against Liverpool's most bitter foes by guiding his team deservedly into the last eight of the competition.
"This victory will resonate long after the quarter-finals have been played, however. After Arsenal were vanquished in midweek, United's chastening experience must surely silence those who doubt Liverpool can provide a genuine threat to Chelsea's domestic supremacy in the coming seasons.
" I t was another step in United's slide and Liverpool's upward curve. The signs had been there at Old Trafford four weeks ago, when a dominant Liverpool were only undone by an uncharacteristic lack of belief and some shoddy last-minute defending to gift United an undeserved triumph."
Bad luck? Five fortunate minutes? It was pure deflection from Ferguson, who had watched his side get outplayed in midfield months after a row with Roy Keane saw the Irishman depart Old Trafford after playing his final game for Manchester United in a 0-0 at Anfield.
Instead it was Dietmar Hamann and Mohamed Sissoko who dominated proceedings in the middle of the park, helping Liverpool control the game after Crouch's 18th-minute opener.
The goal was Crouch's finest moment in a Liverpool shirt since joining from Southampton in a £7m deal the previous summer, having enduring going 19 games without scoring for his new club before netting against Wigan the previous month.
"When we decided to sign Peter [Crouch], we were not talking about signing the best striker in the world," Benitez said. "We were thinking about signing the striker we needed.
"It is the same with Momo Sissoko, Bolo Zenden and Pepe Reina. Sometimes another team with more money will spend big on players that they will not use. We try to use the money that we have on players that we need."
Indeed, all three would play their part as the Reds went on to lift the trophy in Cardiff four months later.
And give Ferguson yet another reason to grumble.
