Continuity mistake: When the detective is in Patrick's office questioning him about Paul Allen's disappearance, there is a bottle and a glass on the table. A glass ashtray appears by the bottle and the glass then disappears between shots. (00:34:20 - 00:35:30)
Continuity mistake: When Bateman goes crazy and flees into his office, his hair is very messed up. However just after he sings and goes to his office, it is quite obvious his hair has been redone and gelled again.
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.
Continuity mistake: When Bateman goes to Paul's apartment to pack up his stuff to "send him to London" after killing him, as he is packing the bathroom light is off and then about 2 seconds later the light has come on by itself with him nowhere near the room.