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 will reading scene after the lawyer has finished, Dibbs' hands are folded on the table, then off it in the next shot.
Continuity mistake: When going through the Up and At 'Em Machine, Kat is splattered with toothpaste. It remains on her face until the blow-dryer segment, and it's suddenly gone.
Chosen answer: It was most likely Pneumonia, which was very deadly before the discovery of antibiotics.
Donald Jenkins