Continuity mistake: In the scene where Kat is being introduced to her new classmates, Casper is seen tying the students' shoelaces together. This includes the snobby blonde sitting in front. But when the camera pans out, her feet are clearly too far apart for this, not like how they showed them while being tied.
Continuity mistake: When Dr. Harvey sees his dead wife, Amelia, for the first time as a ghost, he talks to her half way up the staircase in the manor while the party is going on. The camera changes angles a few times from the side of their heads, to Amelia's view of Dr. Harvey. In each different camera angle, his hair changes place, from the side of his head, it slips down in front of his face.
Plot hole: When Carrigan becomes a ghost, if Casper and no one else, including Dr. Harvey, can remember who they were; how does she all of a sudden know everything that has happened that is going on?
Continuity mistake: When the kids take a vote for having the Halloween dance at Whipstaf, the blonde girl is standing in front of the class as all the kids raise their hands and the bell rings. The shot then goes to the bell and then back to the kids sitting in their chairs. The blonde girl is sitting in her seat when a few seconds ago she was standing up. Also when all the kids get up to leave and trip, we see that none of their shoes are actually tied even though Casper tied them a few moments earlier.
Other mistake: As the priest walks out of the haunted mansion with his head twisted, you can see the head and body don't step in time together.
Continuity mistake: In the scene when Dr Harvey is fighting with Stinky, Fatso and Stretch, he gets smelled out and rolls up in the red carpet down the stairs. When he unrolls out of the carpet, it is obviously a much shorter carpet.
Continuity mistake: At the part when they are in the closet, the light bulb is swinging back and forth and in the next shot it is either stopped or it is gone, one of the two.
Continuity mistake: Kat has different rings all the way through the movie. Like when she's hugging her dad after he became human again, she is wearing one ring on her middle finger. Then on the next shot, the ring is gone.
Visible crew/equipment: In the scene where Bill Pullman is in his office room, in the mansion, the 3 ghosts lift him in the air, you can see the wires lifting the chair.
Continuity mistake: When Kat finds the button in the book it is covered in dust. When the camera angle changes it is wiped clean.