I first heard this story on an episode of QI (series L, episode “Landmarks”).
In 1902, King Edward VII of England was crowned, about a year and a half after the death of his mother, Queen Victoria. You can watch a silent film documenting the event here:
One problem, though. The man in the video is not Edward VII.
The man in the video is a French lavatory attendant.
Let’s back up: in the extremely early days of cinema, France was really the world-power of film making. French director Georges Melies (who directed A Trip to the Moon also in 1902) sought permission to film the actual coronation of Edward VII, which was granted.
That is, until the officials organizing the ceremony heard how loud the film making equipment was, and promptly banned him from Westminster Abbey.
Frustrated, Melies decided to re-stage the entire coronation on a sound stage in France. He managed to stumble across a lavatory attendant who–at least from the front–looked extremely similar to Edward VII.
Edward VII came down with appendicitis on the day of the coronation, which enabled Melies to arrive early and film the actual carriages arriving for the ceremony and splice that into his re-staged footage. Melies worked closely with those involved with the coronation to make sure he got all the details as accurate as possible–with a few exceptions.
Firstly, Queen Alexandra was much taller than Edward VII, a fact of which the husband was extremely sensitive. Organizers quietly instructed Melies to make sure that whatever actor and actress he got, the actress must be shorter.
Secondly, the film version was a bit more ceremonial than the actual royal ceremony, because Edward had been ill so they had cut a few unnecessary elements out of the actual coronation.
Thirdly, because this was a film, events went a great deal more smoothly (since they had more than one take). At the actual coronation, the Archbishop of Canterbury (who was extremely old and nearly blind) put Edward’s crown on backwards. When the Archbishop knelt down to swear fealty to the king, he wasn’t able to get back up again. Edward had to help him.
The film, meanwhile, was a massive critical and financial success. The man who played Edward, whose name I can’t find anywhere, briefly became one of the biggest stars in the world. He reprised his role as Edward VII in Melies’s 1907 film, Tunnelling the English Channel. Edward VII saw the coronation film when it came out, and was reported to have enjoyed it immensely. Melies’s great-great-granddaughter, Pauline Melies, still has a letter of thanks from Edward VII to her ancestor.