The date for Jane's ball is shown onscreen as being scheduled for a Monday. However in a scene set on a day that is stated to be the day before Jane's ball, Jimmy gets dressed to go to work. Jimmy is a banker. The day before the ball would have been a Sunday, when his bank would have been closed. Another clue that the filmmaker forgot which day the ball was on is that Jimmy complains he hasn't slept since Sunday.
In "The reluctant debutante", Angela Lansbury's daughter says to Sandra Dee, "let's see the changing of the guards at Whitehall". The changing of the guards takes place at Buckingham Palace since 1837.
Early in the film, Clarissa and Jane ask their parents (Jimmy & Sheila Broadbent and Mabel Claremont) to be let off at the "Changing of the Guard" to take photos. They are let off to walk home afterward, however Jane just arrived from the United States and neither girl has been to the Broadbent's flat before.
Lady Broadbent incorrectly says "from A to Zee". A British person would use the correct name for the letter 'Z' which is "zed" not "zee".
At the end, when David Parkson says hello to Jane Broadbent in the receiving line of her party, she calls him Nick instead of David.