Revealing mistake: When Ami's chest is being drilled by the "drill-bra" at the end of the movie, you can see blood flowing from her chest, obviously because of the damage the drills are causing her. Yet, later, you can see absolutely no cuts or marks on her breasts or upper chest, where the drills were penetrating her skin. The skin is simply soaked with blood.
Revealing mistake: Whenever Ami uses her machine-gun arm, it is very obvious that the muzzle flashes were added in post-production, as they have a very "animated" look. In addition, her gun never expels any spent rounds, which would jam it eventually.
Revealing mistake: Twice when you see someone with a slit-throat (the maid at Ryugi's home and then later on the guard that Ami kills), you can easily tell the slit-throat is just an appliance, as it is a bit bulky and sticks out slightly from the rest of the neck. Also, neither wound is bleeding, both simply have some blood painted on around the cut.
Revealing mistake: When the final bully is shot in the head during the first scene, when you see the closeup of his head being blown away, you can see some steam/fog from compressed air used to blow out the fake blood.
Revealing mistake: When Ryugi cuts off Miki's foot with his weapon near the end, you can see the leg was a prop, and that not only had it been pre-scored to tear apart, but that the rip was actually about four inches away from the blades of the weapon, meaning it was being ripped off at the wrong spot.
Revealing mistake: When Miki is slicing the man in half vertically with the chain-saw near the end, you can tell that the man is a dummy and is already pre-scored, and that the chainsaw is simply being run-along the premade cut.