The video, which reportedly was filmed at Windsor Castle, shows a tourist mimicking the march of a member of the Queen's Guard. Someone off camera can be heard joking that "his gun is jammed", implying that the tourist had no reason to worry that he was mocking a man with a huge gun.
However, it all becomes too much for the guard when the tourist takes it to the next level and puts his arm on the shoulder of the soldier. The guard instantly turns to towards the tourist and shouts, "Step back from the Queen's Guard."