Continuity mistake: When McClane (Willis) approaches the dump truck in the aqua duct, the driver pulls an Uzi submachine gun onto his lap. The camera then cuts to McClane being seen in the door mirror of the truck, then cuts again to the cab being shot full of holes by McClane. The gun on the driver's lap changes to an MP5K (in this case, a Daisy MP5K Airsoft Replica).
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.
Continuity mistake: Near the end, after the huge gold heist, Simon Peter Gruber and his psychotic lover, Katya, engage in rough celebratory sex, ripping off each other's clothing, such that Katya's upper torso and black lace brassiere are fully exposed. A few minutes later, when John McClain interrupts their sex, Katya's shirt is fully closed.
Visible crew/equipment: After the school evacuation scene starts the four kids trapped in the school look out the window to see what the commotion is. The next shot through the window grill shows a camera crew filming the evacuation from the street as they come up behind the cops on the upper right. Footage from the camera crew on the ground is proof of this. The footage is from the exact same time, about 30 seconds earlier in the movie. You see an African American boy and woman running off to the left of the screen through the police who are trying to guide everyone straight out from the stairs past the fence. The boy continues running but the woman is grabbed on her left arm by a cop who turns her back around to go through the fence. The boy is wearing a yellowish orange shirt and tan shorts and the woman in a brown outfit, a blouse with white buttons and pants. (01:40:10)
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.