Other mistake: Magnum and Vivian are not listening to the TV at loud volume (they have a quiet time watching and talking without raising their voice), but Higgins, all of a sudden hears it all the way from the main house and is greatly bothered by it.
Magnum, P.I. (1980)
1 other mistake in Letter to a Duchess - chronological order
Starring: Tom Selleck, Roger E. Mosley, John Hillerman, Larry Manetti
Revealing mistake: One of the party guests opens the door of the fridge and you can see behind the set instead of looking into the fridge. (00:32:40)
Mac: Time has little to do with infinity and jelly donuts.
Trivia: Tom Selleck is the only actor to appear in every episode of the show.