Continuity mistake: When Princess Mia is being crowned Queen, we see Charlotte at the top of the left set of stairs, the ones Queen Clarisse just descended. We see Charlotte's legs at the top of the stairs until they go out of frame as Mia turns around to stand in front of throne. The next shot is of Nicholas and CHARLOTTE. Wow, how did she get down there so fast?
Continuity mistake: When the newly engaged royal couple is presented to the Genovian public, they walk out onto a balcony between two royal palace horn players. Just as the royal couple begins to wave, the horn players lower their horns. The view is switched to a long shot of the front as the royal couple is waving and the horns are back up. They remain up near the players mouths for the rest of the scene even though they don't play anymore.
Continuity mistake: During the parade scene, there is a man with a full beard/moustache (sort of a reddish brown) with a black hat, white shirt and black jacket that I believe we first see handing Genovian flags to Charlotte before she tries to get Viscount Mabrey to wave one. This man then goes to stand behind the head of the black man sitting next to Viscount Mabrey. Next, after the parade starts, we see Elsie and Miss Genovia at a table and right above and a little to the left of Elsie's head we the same bearded man. We see some parade shots and then another shot of Elsie with the bearded. Then we see Carolina shots and the parade being stopped. The next shot is of Andrew, Nicholas and Viscount Mabrey and just above and a little to the left of Mabrey's head is our bearded man. Then Elsie ends her commentary with the bearded man's chest and flag in visible in the background. Unless he is a twin like our palace guard, he certainly gets around very fast.
Continuity mistake: After Nicholas has kissed Princess Mia in front of the fountain, we can see the front of his hair is messed up, probably from the brim of her hat. As they walk around the fountain, his hair is still messed up. As the scene angle changes as Nicholas says, "Maybe I am," his hair is now fixed and not messed up as in previous shots.
Continuity mistake: When Nicholas is trying to get Princess Mia's attention, he is throwing pebbles at her open window. She sticks her head out of the window. The next shot is Nicholas throwing another pebble and acting like he hasn't seen her. We hear the pebble click against glass but if it actually did, it would have had to almost hit Princess Mia but she acts as though nothing has happened.
Continuity mistake: In the chicken scene, as we see Tiny starting to hand the chicken in a basket to Princess Mia, in the background are Brigitte and Brigitta. They are behind the palace guard and to the left of Charlotte and the housekeeper. When we hear Princess Mia say, "May I?", the scene show Charlotte (and the housekeeper) but now Brigitte and Brigitta are behind Charlotte.
Continuity mistake: When Viscount Mabrey and Nicholas are at Mabrey's home, plotting Nicholas' next move, Mabrey is sitting on a sofa with his legs crossed at the knee and a magazine is across his lap. Mabrey's right hand is holding the top of the magazine. Mabrey points his left index finger at Nicholas and asks if he remembers who his father named him after. The shot switches to Nicholas saying that he was named after Grandfather Nicholas. It then goes immediately back to Mabrey starting to get up but now, in just a second or two, Mabrey's feet are both on the floor and the magazine is folded up and held in his right hand.
Continuity mistake: When Lionel is speaking to the Queen, she introduces him to the housekeeper and her two ladies' maids; they are standing in front of the window directly to the right of the Queen. Lionel leans over to tell the Queen he would take a bullet for her and she says that interns don't even want to get her tea. Joseph grabs Lionel and we see the window where the housekeeper and maids were just standing but they have disappeared.
Continuity mistake: Right after Princess Mia and Andrew meet and are walking on the beach, a shot of them shows Princess Mia's scarf around her neck from the front with the ends blowing very visibly out to the back. Then it cuts to the press corps and then back to Princess Mia and Andrew. You can see her scarf is now on wrapped around the back of her neck with the ends her coat. Then we see her hair move first as her scarf is pulled (blown off?) from a string or wire pulls it off from the back.
Continuity mistake: In the scene where Nicholas asks if he can help Princess Mia with her archery, he has her take her stance, then says to lower her elbow. We see Nicholas reach up to put his hand on her shoulder (his right hand is seen through Mia's forearm and shoulder because her elbow is still up so high). Nicholas then says anchor her right hand to her mouth, to touch her mouth. Then he says to relax her hand on the bow as he reaches up to put his left hand around her bow hand. We then see Nicholas reaching up to put his right hand on Mia's shoulder again but this time we see it over the top of her forearm because her elbow has been lowered considerably.
Continuity mistake: Near the beginning, at Princess Mia's birthday ball, we see a man in the purple and gold palace attire serving champagne on a tray downstairs. The next shot in the scene has Queen Clarisse introducing Princess Mia and then we see two men dressed in purple and gold ready to open the doors for Princess Mia's entrance - but one of the two men is the same man, who just seconds ago, was downstairs serving champagne.
Continuity mistake: When Gretchen is telling Nicholas that his uncle is up to something, her robe changes from purple to blue and back to purple in three different shots.
Continuity mistake: When Princess Mia is being "scolded" by Queen Clarisse in the Queen's suite, Princess Mia is reclining on a pillow the same color as the sofa and another pillow that is white/cream with fringe. The white pillow with fringe is partially up on the other, darker sofa pillow. Then we see a couple of shots of Princess Mia talking to her grandmother and the white, fringed pillow is flat down on the sofa under Princess Mia, not on the other darker, sofa pillow. After Princess Mia leaves, we see a shot of the sofa and the white, fringed pillow is now back up, partially on the darker pillow of the sofa.
Continuity mistake: When Princess Mia and Andrew are running after her scarf at the seashore, we see Brigitte and Brigitta also running after the scarf, just as Mia says, "I've got it." But as soon as Andrew and Mia grab the scarf, the next shot shows Brigitte and Brigitta facing the opposite direction and holding parasols, standing behind Andrew's parents.
Continuity mistake: After Princess Mia and Nicholas fall into the fountain, we see Nicholas come up out of the water and he is just a little to the right of the statue and his tie is not over his shoulder. The scene switches to Princess Mia's head emerging from the water. When she looks over toward Nicholas, the shot shows him now way over to the right and his tie is now over his shoulder.
Continuity mistake: In the courtroom scene when you see a man sleeping and leaning to the left, but the next shot of him, he's leaning straight up.
Continuity mistake: When Queen Clarisse is getting ready to mattress "fly", we see Asana, Lilly and Princess Mia lined up front to back from the post at the top of the slide to the back which has Mia near the doorway by Queen Clarisse, just as Asana asks, "Is she really going to do this?" Then as Queen Clarisse jumps on mattress, we see a shot of Asana, Lilly and Princess Mia lined up across the view, with Asana on the stairs and probably down a step, Lilly mostly behind the balloons and Princess Mia between post and the Queen, almost where Asana was before. After the Queen successfully reaches the bottom standing, the trio are back to their previous position, cheering the Queen.
Continuity mistake: After Princess Mia says, "That's enough flaming ones for now", Brigitte and Brigitta nearly get hit by another arrow. Then when Nicholas shows up, they are holding fire extinguishers, even though Princess Mia are using regular arrows and has said no more flaming ones.
Continuity mistake: As Princess Mia is coming down the aisle of the church, she starts to hesitate about three rows from the back. As we see her from the front of the church, she is very near the back when she stops to address everyone. But as she hands her bridal bouquet to the gentleman, he is close to halfway down the aisle and we can see a woman in a pink hat next to (behind) him and a woman in a lavender hat right behind her (who is then right behind the bridal bouquet man). Behind them, down the aisle towards the back, is a woman in a blue hat and farther back is a woman in a pink hat. But the next shot is a long shot from the front and the man holding the bridal bouquet is very near the back and there are not enough rows to hold the women in all the different colored hats.
Continuity mistake: When Princess Mia is eating ice cream in the kitchen, a blob of ice cream appears on the left corner of her mouth for one shot, then disappears.
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.