Revealing mistake: When Nicky is hit in the head by an officer, the baton obviously bends off his head like rubber.
Revealing mistake: When the priest is running and jumping down the stairs to the subway, he bounces up again, as if he fell on a mat. (00:15:25)
Revealing mistake: When Nicky shoots goodness at a group of demons, one of them bites off a part of one of the rabbits. In the last shot of that demon, it is extremely obvious that the bunny that he is holding is a stuffed animal. (01:08:00)
Suggested correction: Gonna be honest - I genuinely think the fact the rabbit is replaced by a ludicrously OBVIOUS stuffed animal is kind of an intentional joke in itself. The rest of the effects in the film are very solid for the time, whereas it's laughably blatant that the rabbit is a stuffed toy. The fact that the movie lingers on several shots of it with its button-eyes, unrealistic "cartoon" dimensions, etc. kinda tells me you weren't meant to take it seriously. At best, it could maybe be reclassified as a "deliberate" mistake, but even then, I don't think I'd classify it as such.