Audio problem: When Lizzie is performing at the end of the movie at the awards show, at the part where we see her from the back facing the blue light, her voice sounds like it's changing. It seems like it's someone much older singing, or the synthesizer went wrong. It just obviously isn't Lizzie's voice anymore.
Other mistake: In the scene where Paolo leaves the stage after everyone discovers that he can't sing, his bodyguard Serge stops him and Paolo suddenly loses his Italian accent. Paolo sounds very American when he says "You work for me, Serge."
Continuity mistake: In the scene where Lizzie's parents are seeing her off for her trip to Rome, she first walks away from her family after exchanging hugs and the legs of her pants are rolled up. She then turns around and walks back for a second hug with her mother and when she turns around and walks away from the family a second time, her pants are no longer rolled up.
Revealing mistake: Lizzie has black nail polish and the same size of nails throughout the entire movie. Supposedly, Lizzie and Isabella are different people. But, when Isabella comes back, you can see she has the same color nail polish and the same size of nails as Lizzie. I don't think it's just a coincidence.
Continuity mistake: In the scene when Lizzie and Paolo stop to have a look at the view, Lizzie's hair is braided. In the next scene, when they go to the designer's place, her hair is tied up. If her hair was braided, she wouldn't have been able to tie her hair up that easily. In the next scene when she did her cartwheel outside and said "Good-Bye Lizzie McGuire, Hello Fabulous" her hair is braided once again.
Factual error: In the scene where Ms. Ungermeyer returns to the hotel and takes the elevator upstairs, Sergei detains her in the elevator (to give Lizzie time to get into her room) while the classmates, including Kate, walk up the stairs. Since the rooms were on the first floor, Kate should have reached her and Lizzie's room before Ms. Ungermeyer; however, they enter the room at the same time.
Continuity mistake: When Paolo and Lizzie are in the rehearsal hall and Paolo is teaching Lizzie to dance, he is sitting of a chair with a remote control in his left hand, giving her directions. He then stands up to show her a move. He puts the remote down on the chair and stands up. During the move that he shows her, the remote reappears in his left hand. (01:05:15)
Continuity mistake: At the designers, when the dog was taking away the skirt, in one shot the skirt was short, but in the next it was long again.
Continuity mistake: At the end of the movie, you have Lizzie McGuire singing in front of thousands of people. In one shot you can see all the fans as if they were standing on the floor all the way to the walls on the opposite side of the Colliseum, but in the next shot it looks like it is a totally different seating plan, as the seats are going up.
Continuity mistake: In the scene where the group are waiting in the lobby for their room assignments, Lizzie's hair has noticeable kinks where her pig tails were for the flight but when she's upstairs her hair is perfect and shiny and straight. She wouldn't have had time to change it walking upstairs.