Continuity mistake: When Bryce pulls out Paul Allen's card to show it to Patrick, he holds it in different ways between shots. (00:19:44)
Continuity mistake: When Patrick Bateman murders Paul Allen, the collar on his raincoat is either up or down, depending on the shot. (00:27:49)
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: At the restaurant, Patrick approaches his lawyer to talk about the answer machine message he left him and the lawyer puts an unlit cigarette between his lips. The cigarette is already lit in the next shot when he mistakes Patrick for another colleague. (01:31:16)
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.
Continuity mistake: In the office scene where Paul Allen walks up to Patrick Bateman and mistakes him for Marcus, Paul crosses his arms as he starts talking to Bateman. The angle changes and his arms are suddenly straight.
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.
Answer: There are two possible answers. First, Carnes doesn't recognize Christian Bale as Bateman, instead thinking he is someone named Davis. He insults Bateman and thinks "Davis" left the message as a joke. Realize also that someone claims to have seen Owen/Allen in London, but mistook Herbert Ainsworth for him. Second, Bateman is delusional and only thinks he killed Owen/Allen, who is in fact alive and in London.
Greg Dwyer