Continuity mistake: When James and Kat first enter Whipstaff Manor, they drop their stuff on the floor. The large suitcase falls over on its side, but it cuts to the overhead wide shot and the suitcase is standing upright (the ghosts didn't do it). (00:19:45)
Deliberate mistake: After James and Kat enter Whipstaff and get the power turned on, when Kat heads to the staircase to go search for a bedroom, we can see a few of the metal stair runner rods attached to the stairs to keep the carpet runner in place. However, when James falls at the top of that staircase and gets rolled up in the carpet runner like a sushi, there are zero stair runner rods. (00:21:30 - 00:31:50)
Continuity mistake: After Kat finds a bedroom for herself, the long sheer white curtain hanging near the framed photo of Amelia repeatedly changes between pulled open or closed; particularly noticeable when Kat dumps her clothes from the black trash bag and tosses a ball of socks into Casper's mouth. (00:25:15)
Other mistake: While Dr. Harvey is looking at his multiple reflections in the mirror over the sink, he sees the Crypt Keeper reflection. It then cuts to a close-up facing the real Dr. Harvey as he freaks out, and it's a flipped shot—note his shirt and sweater are backwards (this is not a mirror's reflection)—right before he gets his foot jammed in the bucket. (00:31:05)
Deliberate mistake: After Stinky delivers his "Smell-o-gram" to James, he trips over the vacuum and falls, then continues down the stairway, with his body rolled up in the stair runner as it gains momentum. The problem is when the sushi runner hits the ground floor, the runner's edge is facing the opposite way to what it should be; deliberately done for the momentum and to have it humorously unroll. It's entirely impossible for it to turn itself around and then still have even more momentum to unroll the way it does. (00:31:45)
Continuity mistake: Carrigan decides to make Dibs a ghost, so she goes after him with a double-headed axe. Then, when Carrigan slips on the muddy goo, her black pointy toe high heels turn into beige flat rubber-soled shoes, seen in her close-up as her foot lifts up. (01:11:45)
Continuity mistake: After Amelia grants Casper's wish to be alive (albeit for only a short time), when it cuts to Kat sitting alone at the party, note the distinctive worn chairs beside Kat. In the following shots, those chairs not only move their positions, they also switch places with each other more than once. (01:27:50)
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.
Revealing mistake: Casper is shown tying together the shoelaces of the students in Kat's class. When the bell rings and class ends they all get up and fall down. If you look closely you can see that their shoelaces are not tied 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.
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: 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.
Visible crew/equipment: When the priest arrives and when he later walks to the house, lighting screens are reflected on his glasses.
Revealing mistake: 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: 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.
Continuity mistake: When Kat finds the button in the book it is covered in dust. When the camera angle changes it is wiped clean.
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.
Revealing mistake: When Kat goes through the Up and At 'Em Machine, the top comb makes contact with her head, and a few of the comb teeth slightly bend. Thereby revealing that the teeth on the comb are made of rubber.
Continuity mistake: In the will reading scene after the lawyer has finished, Dibbs' hands are folded on the table, then off it in the next shot.
Chosen answer: It was most likely Pneumonia, which was very deadly before the discovery of antibiotics.
Donald Jenkins