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.
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.
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.