Continuity mistake: There wasn't much blood noticeable on the axe after it was used to chop off the mermaid's tail, but a little later when the man was carrying it over his shoulder, it was heavily covered in blood and dripping. (00:14:17)
Continuity mistake: For a second time in the movie, someone buzzes June on the intercom. She tries to overcome her fears and get out of the apartment, but can't. She shuts the door, leans with the left hand on the doorframe by the chain, and lowers her arm. In the next shot, Naomi Watts is breathing deeply still with her hand by the chain. (00:06:05)
Continuity mistake: The second time Tree wakes up, she does a mock "gunshot" to her forehead using her fingers. She then turns her head downwards. When the camera cuts, she instantly goes from looking down to looking to her right. (You have to look closely since only part of her head is in frame, but it's definitely noticeable).
Continuity mistake: In the prologue, the Nazis storm the Bresson residence. The elder Professor Bresson is sitting at his desk, back turned to them. There's an open fountain pen next to him, right by a paper sheet, but when Bresson answers rather defiantly to the bad guys, the paper sheet suddenly has disappeared. (00:02:15)
Revealing mistake: The structure/set-up of Richard's house (front view) doesn't make sense and can't be real. The half of the house to the right of the screen is at an awkward angle and has no windows. What little of the roof that can be seen is flat - not conducive to snowy Massachusetts. The bottom of the door to the right of the garage door is inches above the driveway pavement (elevated with no step). The deck wall to the right of the yellow door is higher than the door but the front rails only reach halfway. (00:03:25)
Revealing mistake: Obvious use of green screen for buildings across the street. On and off, the background fades, and buildings appear higher or lower depending on the camera's angle. At one point, the angle of walls behind Rex (Bruce Willis) is at a downward angle, and his head is almost touching the ceiling. (00:06:10 - 00:07:25)
Other mistake: The Burglar (while tied to a chair and talking about "My Darling Clementine") says, "Apex of Ford's art, you feel me?" Vikar (James Franco) responds, "I feel you." The movie starts in 1969, but rapper E-40 (Earl Tyrone Stevens, Sr.) - "slang master" - supposedly created the expression "you feel me" in the early 1990s. (00:19:53)