Continuity: In the club scene where Young-taek kills the lesbian, he grips her head below her chin and starts cutting her ear. In the next shot, a close up, his grip on her face and the knife has suddenly changed and he hasn't started to cut her ear off yet.
Revealing mistakes: When Detective Kim finds the severed finger, the actor didn't hold the pose long enough, so the frame is noticeably frozen to lengthen the shot.
Audio/visual unsynchronized: The English subtitles on this movie are horribly inaccurate. For example, instead of saying that the killer "cut out" or "removed" the fetus, the subtitle states, "he excavated the embryo."