The film is set near Liechtenstein but all the cars including the taxi have British number plates with the exception of the Daimler limousine.
Many of the titled women are not wearing tiaras, including Lady Arabella's mother, at the Christmas Ball but Lady Arabella and Maddie are unmarried women and would not be wearing a tiara. Unmarried women generally do not wear tiaras and all married, titled women would wear tiaras at White Tie events. A call to the Buckingham Palace Protocol bureau confirmed that unmarried women would not wear tiaras even at state or formal events and untitled women would not wear tiaras at any state or formal events.
When Jules looks at the airline tickets, it says they're in First Class but the seat assignment is way back at 26B. There's no way First Class would be 26 rows back from the front of the plane.
Ashton is the younger brother. His older brother Charles is the rightful heir to his parents' titles. However, when Charles married the children's mother, he had been disinherited, nullifying his claim to the throne. As a part of this is shown in the duke's attempt to get back in touch with his lost family, it becomes obvious that Ashton is, in fact, next in line.
Ashton is wearing a black waistcoat with white tie and tails. The waistcoat should be white for white-tie.
When Prince Ashton is playing the violin, Debussy's "Claire de Lune" is heard. The actor's fingering on the violin's fretboard and his bowing are completely out of sync with the music and with the notes. He should be using vibrato which would synchronize with the music and the actor is fingering the fretboard too quickly and touching notes which would not produce a vibrato.
Although Jules (Irish), Maddie (English) and Milo (Canadian) are portraying Americans, there are several times you hear their native accents slip, typically when they're upset. Most notably when Maddie says "he can stay with us!".
Upon formal introduction, the Duchess of Belmont greets the Duke of Castlebury as "Sir Edward" rather than "Your Grace". Even during informal conversation, he would be addressed as Castlebury or Edward.