Revealing mistake: As Drew Barrymore is running from the killer outside he stabs her. If you look closely it is very easy to tell it is a rubber knife, because you see it bend off of her. (00:10:20)
Revealing mistake: When the guy that was shot in the chopper is falling, you can see the protective, yet flaming mask on his face.
Revealing mistake: When Kiki's legs "reverse" at the end of the film as he runs off, watch closely. It is clear the effect was achieved through some shoddy image-manipulation. His clothing and hands "warp" slightly a few times around his legs.