Continuity mistake: During the scene right after Amy's boss drives away, you can see the street behind her, over her shoulder, is empty of vehicles. The camera zooms in as she flicks her cigarette away, and in the cut shot, suddenly a taxi appears over her shoulder, which was not there a second before.
Continuity mistake: Just before, and during the scene where Amy is watching the video from the dead girl's apartment, in her hotel room, the travel size gin bottles change from laying flat, to one standing, to both laying flat again. This happens three or four times during the scene. Her cigarette lighter is also visible, then not seen, then visible again, even though the camera angle should show it on the table near the glass and gin bottles.
Other mistake: When Amy first goes into Marla's apartment, she sees that the food in the kitchen is spoiled; there is a few weeks worth of mould on the scattered dishes and the food on the plates by the sink is especially bad. Yet on one plate by the sink, there is a piece of bread still perfectly white and free of decay, though it is surrounded by other mouldy scraps. The shot is only visible for a moment.
Continuity mistake: After Amy has suffered the knife wound in her back, in a few scenes she stands still for a moment and blood pools very quickly around her feet; yet each time there is barely any on her clothes, especially her legs where the blood would have had to run down to meet the floor. The notable scenes are in the street after the stabbing and in the institution when she sees the wooden door after Marla disappears.