Basically, Jose Luis Rodriguez Zapatero was summing up at the Ibero-American Summit in Chile and naturally referred to recent mainland Spanish history. In the process, he again naturally referred to the previous President of the Government (Prime Minister) José María Aznar.
Chávez, as is totally normal for him whoever is speaking, started shouting at Zapatero that he shouldn't even be talking about Aznar because he was a fascist and fascists aren't human. Zapatero tried to say, and repeated afterwards, that regardless of the genuine disagreements and dislike between himself and Aznar, Aznar was a democratically elected President of the Spanish Government .....
The microphone was switched on for Zapatero, so Chávez couldn't be easily heard, but he's loud enough for anyone's taste, so the king (sitting next to Zapatero and about five seats away from Chávez, leaned forward and told Chávez "why don't you just shut up". He didn't of course, and minutes later, the King walked out of the room in the middle of the final plenary session. Unprecedented in diplomatic circles and huge news in Spain.
Today, Chávez is spitting fury that he should have been spoken to in such a way by "just another head of state".
There's a report in English
HERE I think JAnot's particular point, though, was the language used, because the King said "Shut up" (which is pretty "informal" language in any circles) to someone who is, after all, a Head of State, and moreover used the familiar 2nd person form "Por qué no
te callas". This doesn't come across, obviously, in a translated report.