Other mistake: Right after Sam tells Carla he will sleep with the next woman who walks through the door, in the background there is an "Extra" (blonde-haired woman seen in the background behind Carla) who starts to cross right then turns and crosses left then turns right and stops and looks confused as to where she is to cross. Obviously she was given directions of which way to cross and when, but forgot. (00:04:15)
Other mistake: Just as Brian Damage says, "Oh, weren't ya?" to Neil, you can see the shadow of Neil's mother on the front door, awaiting her cue to come in.