In the first movie we learn that Genovia Independence Day is before Mia's birthday, but in this movie it is after.
In the "Dancing footman" scene, it cuts back to the interior of the room the footman is dancing outside of. The same footman is standing directly behind the Queen. Then it cuts back to the dancing footman in the hallway.
In the first Princess diaries movie the palace is gray stone but in the second princess diaries the palace is red stone.
During the meeting with the Parliament members, Lilly is wearing a dark pantsuit. After they leave the meeting area, her shirt changed a magenta t-shirt, with no evidence that she changed clothes.
After Mia falls in the fountain with Nicholas her lipstick has rubbed off but in the next shot it has been reapplied.
The Peace Corps is only available to US citizens, therefore, Europeans could never have been in the Peace Corps.
Lady Elyssa is said to have won a Rhodes scholarship, but the Rhodes scholarships are only for citizens of American and Commonwealth countries -- British nationals are not eligible -- to study in England, and none of the nations eligible to send students have hereditary nobility.
At the wedding, when Mia addresses the members of parliament, saying why she wants to rule without a husband and why she would be a great queen, her friends from America are shown watching her live on TV. However, there is clearly no camera crew inside the church where the wedding takes place, and nothing suggests the event is being broadcast.
In the beginning of the movie (at about 00:01:49) the princess is leafing through her diaries. On one page you can see a 20 EURO bill taped to the paper. It's a counterfeited bill, or better a kind of fantasy-bill, composed of parts of the real bill.
When the Queen has an audience with her subjects a farmer hands Mia a basket with the promise of fresh eggs for her table. The farmer declares "She's my favorite" but the bird is obviously a male rooster that doesn't lay eggs.
In the scene where Paolo is arranging Princess Mia's hair for the wedding, he states that he is Italian, that "all Italian men love babies." In the same scene, just before revealing Mia's last hairstyle, he counts off, "Un, deux, trois." This is French, not Italian, Paolo may be Italian but that does not mean he can't speak french.
While Mia is exploring the palace, a session of government is seen. A man is sitting at a laptop, supposedly taking notes. His fingers never leave the top row of keys, and don't move fast enough to keep up.
Mia is shown practicing archery. In more distant shots, the string of the bow can be seen. In close-ups, however, the bow has no string.
When Lily comes out of the closet, after all the squealing and such, Mia says, "You're blond." Lily is clearly NOT blond.
When Clarisse is about to show Mia her renovated suite, she talks about a cousin that did the plumbing. Her line is "Nepotism belongs in the arts, not plumbing", but you can hear the last part of the line is ADR added in later as Clarisse's mouth stops after "arts".
In the scene where Nicholas is helping Mia with her archery, right after he says the line "Can I see you one more time before I go?" His lips appear to still be moving though Mia's voice is heard answering his question. There was most likely different dialog there.
At the wedding, when the security guard is carrying the ring bearer up the aisle you can see the camera and the above light (for the camera) in his shades
Toward the beginning of the film, Mia is standing in the throne room, surrounded by a gallery of portraits of rulers of Genovia. At one point, toward the back of the hall, a famous state portrait of Martha Washington can be seen on the wall of Genovian royalty.
When Mia is practicing The flaming arrow bit the ring target was about 3 or 4 foot off the ground at most but when Mia does it for real at her coronation the ring is more then 30 feet off the ground.
Andrew Jacoby is referred to as having a Ph.D. from Oxford. Oxford does not issue a Ph.D., rather it issues a D.Phil.
When Nicholas says he danced with Mia at the Ball for only about a minute, Mia tells him it was more than a minute. But it actually was 25 seconds.
In The Princess Diaries (2001), Mia's teacher is Patrick O'Connell. In this sequel, his name is referred to as "Patrick O'Donnell."
When the Queen is teaching Mia the art of the fan her fan repeatedly flips from the wrong side facing the camera, to the right side of the camera.
At the end of The Princess Diaries Mia states that her mother Helen would be moving to Genovia with her, but in Royal Engagement it shows her and Mr. O'Connell with a newborn in San Francisco living at the firehouse.