Queen Victoria would not have been amused - and neither was Princess Margaret when she visited the set of one of the Carry On films.
The "3rd Foot and Mouth" line up in the Khyber Pass - aka Llanberis
Margaret was said to be furious when she was shown a clip of star Sid James addressing a letter to her great-great grandmother as "Queen Vicky".
The story emerged as a plaque is due to be unveiled in Llanberis, north Wales, where Up the Khyber was shot in 1968.
Research suggests the princess was "incandescent" at such "disrespect".
It was the 16th film in the corny, pun-laden Carry On series, but they were made on famously low budgets, which helps to explain why Llanberis doubled as Afghanistan.
But the laughter seems to have stopped when Princess Margaret - younger sister of the present Queen - dropped in, according to Richard Coombs, of the Wales Screen Commission.
He said: "Princess Margaret visited the set during the filming and was shown a clip including the scene in which Sir Sidney Ruff-Diamond, played by Sid James, writes to Queen Victoria.
Princess Margaret was known for her love of ballet
"In the letter, Queen Victoria is addressed as 'Dear Vicky' and Princess Margaret was said to be incandescent with rage. I think it's fair to say she was definitely not amused."
Princess Margaret, who died in 2002, was aged 38 at the time, and married to the Earl of Snowdon.
Mr Coombs, the commission's north Wales film liaison manager, said she appeared to regard it as "disrespect to the monarchy".
"She didn't like it one iota," he said. "Although she tried to portray herself as easy-going, she did like the trappings of royalty."
Unsurprisingly, Carry On fans did not take similar offence, and Up the Khyber is regarded by aficionados of the series' traditional British earthy humour as one of the best.
The plaque honouring Llanberis' contribution to film history will be unveiled on 30 September by Angela Douglas, who played Princess Jelhi.
Beforehand, Mr Coombs will also visit Peter Rogers, who along with Gerald Thomas was the brains behind the Carry Ons, to give him a version of the plaque.
A path leading to the summit of Snowdon doubled as the Khyber Pass.
Andy Davidson, who works for IBM in Zurich and also runs a Carry On website, said Up the Khyber was always at the top of fans' polls.
"One reason is that it doesn't really feel like a British film at all - it's very extravagant and extremely well shot and it doesn't look anything like Wales."
"After the actor Bernard Bresslaw had finished working on the film, he was in a restaurant with some friends and the Indian waiter came up to him."
"The waiter asked if he had enjoyed working in his country - he was absolutely convinced that it had been shot in India."
The film also has another unexpected link with Wales. The soldiers' costumes originally saw service in a more serious war classic, Zulu, about one of the most famous episodes in Welsh military history.