Revealing mistake: At the end of the chase scene with the Mercedes and the pickup truck, the Mercedes flips on its side and slams into a tree. As the Mercedes falls back onto its roof, you can see a rope attached to the trunk used to pull the Mercedes over.
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 fighting with Targo on the boat, McClane pulls a chain, which trips Targo over. When we see Targo being tripped from above, you can see it is a stunt double - not only is he bigger than the actor playing Targo, but also his hair is grey unlike Targo's brown.
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 the water is pushing the truck in the aqueduct, one scene shows McClane on top of it. The truck and McClane are a different color indicating they were superimposed into the shot.
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 Bruce Willis is searching the subway cars, take note of the passengers. The same people appear in the two consecutive cars. The actors just made minor changes such as adding/removing hats and glasses. The woman who screams is the most obvious.
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: When John is fighting the huge German thug on the boat towards the end, he flips him over his shoulder and on to the steel floor, which slides apart. It is obviously rubber sheets molded and painted to look real. The shot is an overhead shot and only lasts a second.
Revealing mistake: When Bruce Willis and Samuel L Jackson are climbing down the rope (or whatever it is) to get to the ship, the green Dodge falls off the bridge. If watch closely, you can actually see that the truck is tied by another rope in the back, which rips as the truck falls into the water.
Revealing mistake: When Simon is in the helicopter and shoots at McClane, check the metal strips next to his feet and the protruding metal parts where the squibs have been placed to allow for the pyrotechnics.
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: After McClane chases the ambulance in the cab, he is driving the wrong way down a one way street. Notice the tyre marks on the pavement from previous takes. The cab follows them to a T.
Revealing mistake: When McClane is hanging on the steel cable which pulls their car to the river, right when he says "Run, run" there's a brief shot and it's very obvious that the guy is Bruce Willis's stunt double.
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.
Chosen answer: He sees the reflection of his friend's badge number that he remembered from their conversation about the lottery earlier, so knows the badge was taken from his dead friend.
Captain Defenestrator