Prince William broke this royal etiquette rule when he was a page boy and we didn't even realise it was such a big deal.
The Prince of Wales has attended many royal engagements in his 41-year life but when he was six years old, he had a huge role but it in carrying it out, he made a royal schoolboy error.
There are many lesser known royal rules like the Royal Family can only eat cake in this way, and other rules also apply during formal family events too - including weddings.
Prince William was pageboy at the The Society Wedding of Miss Camilla Dunne to the Honourable Rupert Soames at Hereford Cathedral, which took place on 8th October,1988.
But it appears the youngster was distracted with the big role that one of the royal etiquette rules slipped his mind.
While women are required to follow the Duchess slant, in which the knees are placed together and to one side when seated, at formal events it is also considered poor etiquette for men to stand with their hands in their pockets, according to reports by Good Housekeeping.
Prince William can be seen with his hands placed inside both pockets of his white trousers, which he wore with a pair of pink braces and white shirt as he waited around for members of the wedding party to gather.
And while the hands in pockets gives off an informal look, it's something that Queen Elizabeth II often wanted to do but was advised against doing by her mother the late Queen Mother.
But many years later the late Majesty actually broke the rule in an unseen portrait, which is included in the 2019 book The Other Side Of The Coin: The Queen the Dresser and the Wardrobe, by the Queen's dresser at the time, Angela Kelly.
Angela wrote about the pocket portraits, "Her Majesty wanted to be photographed more informally and have the freedom, for example, to pose with her hands in her pockets. The Queen Mother and her advisors had always advised against this, suggesting it would not be appropriate."
Angela explained that the photographer on the day, Barry Jeffrey, tried to explain the shoot but mid-flow the Queen replied, "No Barry, this is how we are going to do it. Just keep the camera rolling."
Kelly writes, "Her Majesty took her position in front of the lens and started striking a series of poses, slipping her hands in and out of her pockets and placing them on to her hips, mimicking the stance of a professional model. I stood in disbelief, the Queen was a natural."
In other royal news, there's a strict royal rule that means King Charles can confiscate his grandchildren’s toys and Meghan Markle’s little known side hustle that helped her pay the bills revealed - and it’s nothing to do with acting