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.
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.
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.
Chosen answer: It was most likely Pneumonia, which was very deadly before the discovery of antibiotics.
Donald Jenkins