Continuity mistake: When Andy and his friends are on the loading dock smashing the bulbs, there is a white mark from a smashed bulb on the dock next to Cal. But in the next shot the mark has vanished, and Cal smashes another bulb in the exact same place, producing the white mark again. (00:30:45)
Continuity mistake: On the 20th date, Trish and Andy are sitting on a chair discussing the date and that they are going to have sex. When Andy speaks, her right hand is on his top button. When Trish speaks, its back in her lap. This happens three or four times, all in the same conversation.
Continuity mistake: During the scene where Beth is in the bathtub, all of Andy's shots have Beth's foot in the lower right hand corner, even though she only has her leg up for portions of the scene. Eg: when Beth puts her "friend" in the water, both her legs are under; we cut to Andy and the foot is in the shot, and then we cut to Beth and both her legs are still under.
Continuity mistake: During the "Aquarius" montage at the end of the film, Andy's "man-o-lantern" chest-wax job is visible in one cut (with the hair just starting to grow back), then completely gone in the next. Since the bedroom scenes in the song are meant to be from the same night, there's no way that his chest could fill in so evenly so quickly.
Continuity mistake: Before and after the dream sequence where he kicks the girl and gives her a bloody nose, the comforter he wakes up with is completely different from the one he went to sleep with, which matched his pillow exactly. When he wakes up, nothing matches the pillow.