Emphasis on mild. The background to the little video is this: Bobby Fischer repeatedly played the Two Knights Variation against the Caro-Kann in the 1959 Candidates Tournament, with a serious lack of success. It got so bad that people who seldom or never played the Caro-Kann did so, as his results and the positions he received with the Two Knights were harmless at best. Fischer lost with it against Tigran Petrosian in round 2, drew with Vassily Smyslov in round 7 and then lost to Paul Keres - not a Caro-Kann practitioner - in round 8 (and then again in round 22).
So by the time of round 13, when Fischer faced Mikhail Tal with white for the first time in the tournament, this happened:
Tal fakes the move 1...c6 before giving the pawn a shove to c5, then offers a priceless smile to Fischer, who remains impassive. Fischer's non-reaction is a pity. Could he not take a joke (very possible), or was his poker face a matter of competitive strategy? As for the joke itself, it almost wasn't one. According to Tibor Karolyi (page 413 of his excellent Mikhail Tal's Best Games 1: The Magic of Youth) Tal seriously considered playing the Caro-Kann in that game. Fortunately for everyone but Fischer, he didn't, enabling us to enjoy his joke, his infectious grin, and the very nice game he went on to win.
(HT: Brian Karen)