The scarlet tunic-clad Grenadier Guardsman, who has not been named, raised eyebrows – and the odd chuckle – among tourists when he changed his march during what must be a boring routine "watch" outside the palace.
And it was all captured on video.
Queen Elizabeth, 88, and her immediate family were not in residence, but the guard's bosses are thought to be "not amused" by the apparent stunt, which they are now investigating. The Grenadier Guard are the British Army's most senior infantry regiment. Insiders at the Ministry of Defence say they are still looking into the video.
But they warn he could face the music over it. "We are aware of the video," a Ministry of Defence spokeswoman confirms to PEOPLE. "Anyone who is found to fall short of the Army's high standards can expect to face appropriate action."