Other mistake: Jethro said to Brian, "Shall I tell you how I know you're lying? Because when you said that, you looked up to your left." However, Brian looked to his right and Jethro even pointed to Brian's right when he was telling Brian that he looked to his left. (00:56:00)
Continuity mistake: The spacing (almost touching versus several inches apart) and appearance (shape and whether there are shoots) of the two flowers in the vase on Estelle's/Betty's table at the restaurant vary. Sometimes, the flowers on her table look more like the ones that could be seen on the table in front of hers after Brian/Roy entered the restaurant. (00:03:40)
Continuity mistake: Sira pours Sprite into a glass until it is about three-fourths full, but the glass is closer to half-full when he hands it to his mother. (00:02:00)
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: 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: 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)