Revealing mistake: When Louis gives Gage the lethal injection, because the syringe is transparent you can actually see the prop needle retract in and out of the syringe as he supposedly injects Gage.
Revealing mistake: When Louis is fighting Gage at the end, multiple times it's clear he is fighting a doll.
Revealing mistake: When the mother gets into an accident after her tire blows, the ghost shows up. It is floating above her car. Notice the seam where the ghost is on the car. When the scene shakes slightly, the ghost shakes too indicating it was a poorly superimposed shot.
Revealing mistake: When Lewis is trying to get Gage off him, as he's struggling with the scalpel, you can plainly see its not really Gage but a dummy made to look like him. Looks like a doll.
Revealing mistake: When Louis goes to give Gage the shot if you look closely you can see that the needle doesn't leave a puncture mark.
Revealing mistake: When the housekeeper hangs herself, you can tell the rope isn`t even around her neck. It is obviously connected to a harness around her torso.
Revealing mistake: When Lewis gets out of the tub and has a towel around him, you can see he has underwear on.
Revealing mistake: When Louis sets Gage's lifeless body on fire, it becomes apparent that it is a stunt dummy. The body resembles a small, pale mannequin with vague facial features and a wig.
Answer: Because in the book, the sign at the entrance to the "Pet Semetary" was written by some anonymous child. Clearly he/she couldn't spell that well yet.
Grumpy Scot