Jose Mourinho and Pep Guardiola's rivalry began when they were in charge of Real Madrid and FC Barcelona, respectively, but ahead of one El Clasico, the Spaniard sent the Portuguese tactician a foul-mouthed message for the whole world to hear.
Click through the gallery above to see some of Guardiola's facial expressions during a heated press conference in 2011.
Everyone knows Mourinho likes to provoke. Mind games are, or at least were, everything to the Portuguese manager, who believes, or at least believed, matches were already won in pre-match press conferences.
In fact, his stance in this regard was so dogmatic that it was reportedly the reason he did not get the Barcelona job in 2008 ahead of a young and inexperienced Guardiola, whom he was already familiar with from his days as a translator at the Catalan giants. The Portuguese first worked under Sir Bobby Robson, but stayed on as a trainer under Louis van Gaal when the former moved on.
In 2011, it was business as usual for Mourinho when he openly called Guardiola out for complaining about match officials. The then-Los Blancos boss said: "Up until now, we've had two groups of coaches. (There's) a very small group who didn't talk about referees. Then there's a larger group, of which I'm included, of coaches that criticises referees when they make big mistakes.
"And with Pep's statements we've come to a third group, which is a one-person group, who criticises good decisions made by the referee. I've never seen this before."
It wasn't unusual for Mourinho to talk about another manager in his press conference, but it was the manner in which he did so that rubbed Guardiola up the wrong way, prompting an angry response from the then-Barca boss. It was a dangerous move, however, because he was looking for a fight on Mourinho's turf.
Flustered, clearly agitated, and perhaps even drained, Guardiola launched an attack on his managerial counterpart ahead of an important UEFA Champions League match in 2011. Before the first leg of a semi-final with a lot at stake.
He began: "Tomorrow, at 8.45, we will meet each other on the pitch. Off the pitch, he has already won. In this room (press room), he's the f*****g chief, the f*****g man, the person who knows everything about the world, and I don't want to compete with him at all. It's a type of game I'm not going to play because I don't know how.
"I won't justify my words. I congratulated Madrid for the cup that they won deservedly on the pitch and against a team that I represent and of which I feel very proud.
"Off the pitch, he has already won, as he has done all year. On the pitch, we'll see what happens. If you think that his allegation that I always complain about the referees is true after you've all heard from me over the last three years, well there's nothing I can do. Off the pitch, there's nothing we can do to fight that."
On the pitch, Messi scored twice at Real's Santiago Bernabeu to secure a 2-0 away win for the Blaugrana, with the second leg ending in a 1-1 draw. Having won 3-1 on aggregate, Barcelona advanced to the final, and went on to beat Manchester United at Wembley Stadium.
While the football in the final was breathtaking, the closing stages of the 2010/11 UCL campaign will be remembered by many as the moment Guardiola finally stood up to his bully.