Sir Alex Ferguson and Rafael Benitez used their hands for tactical analysis on Sunday, but not for a cordial pre-match greeting.
Gabriel Heinze had a comparatively unremarkable career at Manchester United, but the Argentina international was the genesis for a toxic relationship between Champions League-winning managers.
The 34-year-old defender moved to Old Trafford in June 2004 in a £6.9 million deal from Paris Saint-Germain and helped United win the 2006 Carling Cup and Premier League title in 2007.
But Heinze was dislodged from the left-back role at United by Patrice Evra and wanted a move away from the club.
United executives and manager Sir Alex Ferguson agreed to allow the player's representatives seek out a move as long as it did not involve a Premier League rival.
Heinze and Benitez lost the battle to link up with each other 40 miles down the M62, and the player joined Real Madrid before the transfer window closed in August 2007.
Heinze continued the spat after his move to the Bernabeu Stadium (BBC Sport) but a marker had been laid down between Ferguson and Benitez.
At the start of January 2008, Ferguson backed Benitez after Liverpool co-owner Tom Hicks revealed the club had spoken with Jurgen Klinsmann about the manager's job at Anfield, as the Daily Telegraph reported.
The cordial relationship between the two men was to fracture a year later when Ferguson told the official Manchester United magazine that Liverpool would "get nervous" in the second half of the 2008-09 season in pursuit of the Premier League title (Daily Mail).
It was a classic Ferguson mind-game comment and Benitez, whose team were seven points clear of United and three ahead of second-placed Chelsea, fell headlong into the trap.
The former Liverpool manager's press conference on Jan. 9 is one of the most memorable in Premier League history as Benitez dug deep into his pocket for a pre-prepared list of facts regarding Ferguson, match officials and the fixture calendar.
Liverpool would go on to win 13 points from a possible 24 available following Benitez's press conference, but the Spaniard refused to let the matter go and used his post-match briefing after the 0-0 draw at Stoke City to question the United chief executive David Gill's role with the Football Association (Liverpool Daily Post).
Ferguson belittled Benitez's accusations as "absolutely ridiculous", as The Guardian recorded. And the United boss poured petrol on the fire two months later ahead of the two teams meeting when he claimed "I would need to read more of Freud before I could really understand all that (Benitez's words)" as The Independent reported.
After Liverpool's stunning 4-1 win at Old Trafford, Benitez claimed he had read Freud but needed to work on his English (Daily Mail).
Ahead of United's FA Cup semifinal against Everton in April 2009, Ferguson was again on the attack as he defended both David Moyes and Blackburn manager Sam Allardyce, as the United boss reignited the feud three months later (via Daily Telegraph).
A month later, Benitez refused to congratulate Ferguson when United won the Premier League title (The Guardian).
In June 2010, Benitez left Anfield by mutual consent as his relationship with Hicks and fellow co-owner George Gillett left the Spaniard in an increasingly difficult situation (Sky Sports).
There was no doubt Ferguson and Benitez would meet up again somewhere along the line, and the former Inter Milan manager's appointment as interim manager at Chelsea in November provided the opportunity.
Ferguson was not going to miss the chance to rile his adversary and declared Benitez "very lucky" (BBC Sport) to be appointed ahead of the FIFA Club World Cup after succeeding the sacked Roberto Di Matteo at Stamford Bridge.
The latest episode in the rivalry between the two men left Benitez claiming Ferguson did not shake hands before the FA Cup quarterfinal between United and Chelsea at Old Trafford (The Sun).
For the viewing public, we should be grateful for United goalkeeper David de Gea preventing Juan Mata from settling the tie in the dying moments of the game on Sunday.
Now we will get to see another collision between Ferguson and Benitez ahead of the replay.