Revealing mistake: Near the end, when Glenda says, "That's my name. Don't you wear it out," she bends her left hand back and you can see the joint mechanisms (sticking out from under her wrist) that are used to move her hands. This is odd because Glenda is supposed to be a plastic doll in the film, while Chucky and Tiffany are supposed to be animatronic figures brought to life.
Revealing mistake: When Joan is burned and she flies over the railing, you see it's not Joan but a stunt double. The stunt double is also obvious in other shots in the film.