Other mistake: The engagement ring that Mia gets at the beginning is larger than the one she gives back to him at the wedding.

The Princess Diaries 2: Royal Engagement (2004)
1 other mistake - chronological order
Directed by: Garry Marshall
Starring: Anne Hathaway, Julie Andrews, John Rhys-Davies, Hector Elizondo, Callum Blue

Continuity mistake: When they are looking at prospective husbands, Princess Mia and Lilly are eating popcorn and Lilly asks if it tastes like pears. Princess Mia answers its a "Genovian specialty," and the popcorn disappears immediately afterwards. (00:25:45)
Queen Clarisse Renaldi: A princess never chases a chicken.
Trivia: When Julie Andrews says that she never slides, she adds that she "has done a lot of flying". That is referring to Mary Poppins, which she starred in as Mary Poppins.
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Answer: My guess is because Mia and Michael didn't actually start dating until the third Princess Diaries book. The second and third books were all about Mia trying to get Michael to go out with her, and since they went out at the end of the first movie, they couldn't base the second movie on the second book.