Revealing mistake: When the female terrorist goes to use the large drillhammer on the concrete wall, the force of turning it on causes it to "jump" in her hands. When the drill bit bounces against the concrete wall, you can clearly see the wall move back and forward about half an inch.
Revealing mistake: When Willis is driving through the park, you can see the tyre marks and lines where they had driven there on other takes.
Revealing mistake: When Charlie (the bomb technician) hears that there are still children in the school, he continues trying to disarm the bomb. Pay attention to the shots of the countdown timer. It goes straight from 1:01 to 0:59. There is no 1:00.
Revealing mistake: When they are being chased in the Mercedes, McClane tells Carver to pull the fuse for the anti-lock brakes. When Carver says he won't know which fuse is which, McClane tells him to pull them all. If Carver were to do this, he would cut electrical power to critical functions, such as the fuel pump or ignition system.
Revealing mistake: When the truck is being pulled off of the bridge by the ship, if you look closely right before the truck plummets towards the water, there is no engine, transmission, or anything else in the engine compartment of the truck, which was left exposed after the hood fell off in an earlier wreck.
Revealing mistake: Right after the explosion, when Simon walks up to a detective and introduces himself as an adjuster for the bank, he asks if the detective can show him the way down. When the detective says yes he calls over an officer Murphy, who is facing the crowd. Murphy is already looking back and begins to walk to the detective before he is even called for.
Revealing mistake: Towards the end of the film when Simon and his men are found by McClane (Willis) and Zeus (Jackson), Simon's men can be seen taking MP5K submachine guns. The guns are actually Daisy MP5K Airsoft replicas. At one point, a man "pumps" the forward grip of the gun to cock it. Indicating it as an airsoft gun and not a real submachine gun.
Revealing mistake: When the bomb explodes at Bonwit Tellers at the start, we see a van fly into the air. You can see the devices that throw it upwards.