We all know not to mess with the Queen's Guard -- even if their hats are hilarious -- since they're, y'know, active duty SOLDIERS. Even so, some dummies can't help but mock their rigid marching style and purportedly unflappable demeanor. Until, of course, they go too far and have to be put in their place.
Case in point, the dopey kid in the brief video above. After "marching" along with the guardsman for a few seconds, he makes the critical error of placing his hand on the guard's shoulder, prompting him to grab his gun, point it bayonet-first at the kid, and bark "GET BACK FROM THE QUEEN'S GUARD." Apparently this is standard operating procedure, which should surprise exactly no one since again, they're soldiers.
As for the kid, well, he's probably not gonna be doing that again any time soon.