Revealing mistake: The scene of rioting in the UK features a load of police officers running through a crowd. All the officers shown are wearing black tunics and custodian helmets worn with chinstraps. This is obviously old stock footage from a protest in the 80's or early 90's as nowadays, (and especially in 2012) the tunic has not been worn as general wear for well over 15 years, and Brit cops are now better equipped for protests with body armour, NATO helmets, flame retardant overalls, etc.
Revealing mistake: It is not what is seen here that is the problem, but what we don't see on the Hogwarts Express; first on the way to Hogwarts when Harry, Ron, and Hermione are in their compartment talking about Draco, and second, during holiday break, when Harry and Ron are talking about Unbreakable Vows. As the camera faces the large compartment window, we see the scenery out the window, but as the train speeds along, the scenic reflection of the windows opposite it, in the corridor, should be seen but isn't. What is reflected is the light colored wall of the compartment beneath the blacked-out window. The camera filming the shots from this angle is positioned at that window, and to hide the camera/crews' outline, the window was covered.
Revealing mistake: When John is stuck in traffic and he walks up to the other man asking if anyone was hurt, the guy looks behind John's shoulder as if the plane is coming right behind him. Then John turns around and looks right behind him as well. But the camera then pans all the way to the side to see the plane coming in before it crashes - they were looking in the wrong place.
Revealing mistake: In the hospital scene when the nurse is filling the tub for the patient in the room above the cowboy's, he leaves the water on. When the water starts to come out of the tub it is coming out many times faster than it is going in.
Revealing mistake: When the old couple who give Wolverine clothes are first driving towards their house, there is an aerial shot of them driving over a bridge. This is supposed to be set in Canada (drive on the right) but it was filmed in New Zealand (drive on the left). They are driving on the right but the arrow on the road is on the left side.
Revealing mistake: After Garber shoots Ryder, he is told by the approaching policemen to "drop your weapon". At which point the camera pans to a shot behind Garber where he is holding the gun in his right hand and proceeds to drop it. Watch the bottom of the screen and you will see Garber catch the falling gun with his left hand.
Revealing mistake: Axel's badge says deputy sheriff. He is THE sheriff. Not a deputy.
Revealing mistake: Throughout the movie closeups of Enik's eyes reflect the lights and set around him that aren't part of the movie. During the scene when he's talking to Dr. Marshall about how the crystal controls the sleestack, you can see the white ceiling of the soundstage and lights which instead should be the sky.
Revealing mistake: After Miley lets a truckload of walnuts roll down the street, the British reporter is having a hard time keeping his balance. At one point, both feet just barely touch the ground and it is quite obvious that he was suspended on a cable during filming.
Revealing mistake: In the shot right before Coraline finds the hidden door for the first time, she says "Alright Little Me, where are you hiding?" and drops her notebook and pen on the floor. When the notebook and pen are falling, you can see the wires that were used to suspend them while they were being animated. It's a common stop motion technique, but these days they usually try to paint the wires out afterward. It's a pretty severe dutch angle though, and they're only visible for a few frames, so it's easy to miss. (01:27:55)