The Treason Act of 1702 by which Ralph has Lord Percival Graves arrested is an actual Act of Parliment, enacted in the last year of William the Third's reign. It made it a capital offense to attempt to prevent the successor to the crown from properly taking the throne. Eventually, in 1998, the punishment was changed from death to life imprisonment.
During the film, Ralph and Miranda go to a burger King where Ralph fails to get a Whopper. One of John Goodman's first acting jobs was in a commercial for Burger King where he happily consumes a Whopper.
According to an interview with John Goodman in the article "'No Leading Man,' 'King Ralph' Insists" published in the 12th February 1991 edition of the 'Los Angeles Times', Goodman denied his new leading man status in this movie and said he was still just a character actor. Goodman said: "Don't call me a leading man. This is just another part, with a lot more lines. The only difference for me this time was that I didn't have time to fool around when I wasn't working. When you're the leading man, your meter's running all the time and you're always working. There's no goof-off time. But that still doesn't make me a leading man. I'm still just a character actor. Nobody's ready to call me Mel Gibson Jr., and I don't think anybody's ready to pay good money to see me get the girl in the movie. I know I wouldn't go see something like that".
The real royal family name, Windsor, was changed to the fictional name Windham for the movie. Un-synched lip movements in a few parts (most notably the abdication scene) seem to suggest the change was made after the film was shot.
When first discussing Ralph with the Prime Minister Sir Cedric and Lord Percival refer to the house of Wyndham (Windsor) and then indicate that the next would be descendants of the house of Stuart of which Lord Percival is one. The next would in fact be the Hanoverians.
Clarke Fountain at Allmovie states that the type of heir that Ralph Jones (John Goodman) is in this film is a "collateral heir". Fountain writes that "sometimes a family title, among the nobility of England, goes to the "collateral" heirs - people not in the direct line of decent, like cousins, great-nephews and the like. On rare occasions, these people are not even aware that they are about to be elevated to the House of Lords, and they have been living more-or-less ordinary lives. In this comedy, the family which lacks direct heirs is the Royal Family of England - as all of the likely heirs have died one after the other in swift succession".
In the film's source novel "Headlong" by Emlyn Williams, the Ralph Jones (John Goodman) character was known as Jack Green, becoming King John II instead of King Ralph. The character of private secretary Sir Cedric Charles Willingham (Peter O'Toole) was known as William "Willie" Millingham in the book whilst the character of cabinet secretary Lord Percival Graves (John Hurt) was called in the novel Sir Godwin Rodd, and nick-named "Sir God".
The name of the African state was "Zambezi". The country is fictitious, though ironically, the later animated movie Zambezia (2012) had a a similarly titled name which refers to that film's fictitious African bird city. Moreover, there actually is an African place called "Zambezi" but it is not a country. Wikipedia states, "The Zambezi (also spelled Zambeze and Zambesi) is the fourth-longest river in Africa, and the largest flowing into the Indian Ocean from Africa...The 2,574-kilometre-long river (1,599 mi) has its source in Zambia and flows through eastern Angola, along the eastern border of Namibia and the northern border of Botswana, then along the border between Zambia and Zimbabwe to Mozambique, where it crosses that country to empty into the Indian Ocean".
Although Finland is not in reality a monarchy, it almost became one after independence was declared in 1917. Even a prospective king was chosen, but later the idea fell out of favour and a republic was created instead. Since the film clearly happens in a slightly different alternate history, the appearance of the Finnish royal family should not be considered a goof, but a rather erudite touch from the filmmakers.
The film is loosely based on the 1980 novel "Headlong" by Emlyn Williams. Both Williams and John Hurt (Lord Percival Graves) played the Roman Emperor Caligula in adaptations of Robert Graves' 1980 novel "I, Claudius": Williams in the unfinished film I, Claudius (1937) and Hurt in I, Claudius (1976).
In the picture's source novel "Headlong" by Emlyn Williams, the character equivalent of private secretary Sir Cedric Charles Willingham (Peter O'Toole), William "Willie" Millingham, is crowned King William V at the end of the book, which was first published in 1980 about two years before Prince William Windsor was born in 1982. Assuming William one day becomes King of England, and that he is likely to take the title of William V, this will be the same "William V" monarch title that William Millingham is called in this film's source novel.
When private secretary Sir Cedric Charles Willingham (Peter O'Toole) is crowned King Cedric I at the end of the film, though fictitious, it became the third time in motion pictures that O'Toole had played an English Monarch. O'Toole portrayed King Henry II (King Henry the Second) in both Becket (1964) and The Lion in Winter (1968).
Early in the film, the entire British Royal Family is wiped out due to an electrocution accident. In the movie's source novel "Headlong" by Emlyn Williams, they are not electrocuted, their cause of death is different. In the film's source book instead, Wikipedia states, "in May 1935, the entire British Royal Family is killed in a freak accident after the explosion of a large dirigible (similar to the Hindenburg disaster)".