Revealing mistake: During the driving scene, the background is illuminated by non-existent headlights. It's running backward on the projection screen.
Revealing mistake: During Alex's fantasy after he says, "I knew such lovely pictures", they show a bride being hanged. You can see the "bride" has a beard.
Chosen answer: It's part of the "fun element" of the crime to get the victim to open the door themselves.
Captain Defenestrator