Kings usually move behind cordons, protocol and a carefully measured distance from the crowd. That is what makes the picture from Jedburgh in the Scottish Borders so unusual: King Charles III, at 77, in an immaculate grey suit, standing a few metres from a couple of dozen burly men throwing themselves at one another in the mud over a single ball.

The occasion was an old Scottish custom - the traditional street ball game, which has neither the rules nor the boundaries that modern sport knows. The king took the ceremonial first kick, put the ball into play, and then came the thing no protocol can prepare you for: the players surged toward the ball and ended up dangerously close to the monarch. Security reacted at once - they formed a protective ring around Charles and pulled him back while the tangle of bodies wrestled on the ground.

It ended without incident. The king stayed calm, kept watching and chatting with the participants, and the club president later recounted that "the king couldn't stop laughing at how rough the game was." Beforehand, Charles had joked that there weren't exactly a lot of "health and safety" measures on the pitch.

There is something disarming about the scene. An institution centuries old, trained to control every frame, suddenly comes up against a custom even older than itself - and has to take a step back. Security did its job, but that very stepping back briefly showed the monarchy the way it is rarely allowed to be seen: not as an untouchable image, but as an old man in a grey suit laughing in the middle of the mud.