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: When Ami closes Miki's eyes at the end, as she lays dead you can see a pulse in Miki's neck, although she's dead.
Continuity mistake: When Miki's husband is killed by the shuriken (large throwing stars), they enter and exit his body vertically (you see them embed themselves into the wall vertically), yet when he falls to pieces, the cuts are all horizontal.
Factual error: When the Kimuras make the chef eat his own fingers, you can see that all of the finger is on the piece of sushi. Yet, when we see his bloody hand, his metacarpals are all still attached. (00:37:30)
Plot hole: Ami's arm was sliced off, and the wound went untreated (you can tell it was not cauterized in later scenes) for a number of hours before it was stitched up. All this time, we see her loosing ludicrous amounts of blood. Yet, Miki looses her foot, and dies of blood loss only a few minutes later, despite her wound and blood loss being much less severe than Ami's. An inconsistency in the style done solely for dramatic purposes.
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.
Deliberate mistake: When Ami's school friend is killed, a short sword is stabbed into the top of her head. Less than a half-second later, we see her face is totally coated in blood, and she gurgles even more blood before dying. There simply has not been enough time for such a volume of blood to have dripped down her face.
Factual error: During Ami's first attempt at revenge, when she kills the mother (who is overlooking her dead son's head), she stabs the mother in the back of the head, making the knife exit through her mouth. Yet, for no apparent reason, we see intestines fall out of the mother's mouth, which makes absolutely no sense. The intestines are in the lower abdomen, not in the head/neck area. More than likely done for a stylized purpose, but still a mistake.
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.
Factual error: Despite Ami having her hand dipped into boiling oil and deep-fried, her skin is relatively undamaged in later scenes. The implication is that her skin was saved by the fact that the batter which splashed onto it cooked, sparing her skin, but that is simply impossible, especially given how long her hand was held in the pot. Realistically, her skin should have suffered severe burns.
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.
Factual error: When Ami and Miki are driving nails into the henchman's face to make him talk, it is very clear that the depth of the nails would have killed him, as they would have penetrated deep into his brain. Obviously done for stylistic effect, but a mistake nonetheless.
Audio problem: When Ami first gets her machine-gun arm and is blasting away at the teenage Ninjas (in red), when she blows a hole through the stomach of the second Ninja, you can hear one extra gun-shot after Ami stops firing her gun.
Answer: I will have to answer my question after another viewing of the film. The gun is shattered by the "special" bullets that Miki gave to Ami, which were too powerful, causing the gun to fragment when it was yanked off her arm. The gun barrels fall of and impale Ryugi.