Visible crew/equipment: In the scene after Patrick Bateman's second encounter with the detective in his office, we see Bateman having sex at Courtney's apartment. After he rolls off her, Bateman approaches the large, stand-up mirror in the bedroom. In the upper right hand corner of the mirror, a boom microphone can clearly be seen moving around to pick up Christian Bale's lines. The next time we get the same long shot, the microphone is gone.
Other mistake: In the scene where Bateman is about to murder Paul Allen, he turns on the CD player and walks towards the axe meaning to say "...song's so catchy;" but if you watch his mouth, you can see that in place of the word "catchy" there is a blatant voiceover. Who knows what he said in the original shooting.
Factual error: When Bateman is in his apartment with his secretary, he points a large tool at the back of her head, unbeknownst to her. That is an air powered nailgun for installing plywood. The only way that one could be used is with an airhose and a compressor. The fitting for the airhose that sticks out of the bottom has been removed because it usually would point straight down and stick out about 2 inches.
Continuity mistake: In the scene where Patrick is in his office working on a crossword puzzle before talking to Jean, he is writing with his right hand. When the shot changes to the reflection of Patrick's glasses, the reflection shown is as if he is using his left hand. The reflection isn't a mirror image as it should be.