Revealing mistake: In the New York meteor shower scene, most of the cars blow up without having been hit at all.
Revealing mistake: When Blackout is destroying the base in Qatar, a few Humvees get flipped over, and none of them have engines in them. (00:07:00)
Revealing mistake: When Montoya uses a spear to toss the vampire woman from the prison door into the sunlit street, it is an obvious dummy that has been used. First you see the woman stumble out, and the first flames shoot out of her, then the angle changes to a close-up of her as she is engulfed in fire. In the close-up, she has become bigger and rounder, and does not move.
Revealing mistake: The whole time during the president's escape from AF1 (the scene where he's fighting with his corrupt agent) you can see through the airplane door that there is a loop in the movement of the water. It moves for about a second, and then stops, etc., etc.
Revealing mistake: Near the end of the film, after the shootout, there's a shot with the camera zooming in on a newspaper at Will Smith's front door. If you pause it right once the headline fills the top of the screen, you'll notice all the paragraphs are just repeated, word for word. Additionally the word "received" is misspelled.
Revealing mistake: When Damoscinos is dying, just after being bitten, he stumbles to the ground. As he does so, you can see the tube feeding the green "blood" to his neck on the ground behind him.
Revealing mistake: In the skywalk scene, when McClane pushes the paint stand on the terrorist, it is quite obvious that they use a dummy when the paint stand falls on the terrorist and crushes him. (00:45:05)
Revealing mistake: When Viggo pushes the car off the edge and it hits the ground, as it rolls you can see that the engine/transmission, drive shift and tank have been removed. (01:26:00)
Revealing mistake: As Terry and Martine approach Le Sac shop, you can see reflections in the window of that shop of trees. This gives away the fact this was a custom built set (as shown in the extras) and not the real location in London. (00:20:05)
Revealing mistake: While the guys are going for their physicals, there's a scene in which blood is drawn and you can clearly see the tube connected to the end of the syringe pumping fake blood into the container.