(at arouns 47 mins) The school principal opens the library door and the boy with the fire extinguisher on his back hits him. They are crashing against the lockers, and the extinguisher's foam spreads lightly.
Minutes later, the foam is now onto the door, and the lockers are clean.
In the close-up of Casper, while invisible, pulling the fire extinguisher off the wall, the "Instructions" text on its label is in yellow. However, in the very next shot, in which the extinguisher floats away, the text is in white.
When the ghostly trio were speaking out the word "bulldozer" when they were referring to the construction vehicle. They were wrong, its not a bulldozer it was in fact a front loader.