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When Lucy informs John McClane that there are five villains left, she actually miscounts because there are six: Thomas Gabriel, Trey, Emerson, Rand, the truck driver (Robinson) and the one that would later guard her in the Hazmat truck. This is probably the reason, that after killing four of them, McClane was not aware of Emerson's presence in the warehouse at the end, until he is shot by him. This could be in reference to Die Hard 2 (1990) when after only killing five men (Cochrane, O'Reilly, Mulkey, Sheldon and Thompson), he says there are six dead bad guys.
A water treatment facility near Los Angeles doubled as the film's Woodlawn Social Security Administration building. The facility has miles of underground tunnels, and was also used in Die Hard 2 (1990) when Bruce Willis runs through tunnels under the airport.
The stunt featuring McClane driving a police car into a helicopter took three weeks to rehearse. The shot was accomplished by suspending the helicopter in the air with cables and combining two separate shots: one of the stuntman leaping from the helicopter and one of the car colliding with it. CGI was then used to delete the support cables and add rotor blades.
It took four months to assemble and combine archive footage of past American presidents from Franklin D. Roosevelt to George W. Bush to create the televised warning from Gabriel. The goal was to create a video representation of a ransom note.
The elevator shaft sequence was not in the film's script when Len Wiseman was hired as director. Wiseman added it because he associated elevator shafts and claustrophobic spaces with Die Hard (1988).
Bruce Willis' stunt double, Larry Rippenkroeger, was seriously injured when he fell 25 feet to the pavement. He suffered broken bones in his face and fractures in both wrists. Production was temporarily shut down. Willis picked up the tab at area hotels for Larry's parents and visited him numerous times at the hospital. Larry also doubles for James Caan in his television series, Las Vegas (2003). Caan came and visited Larry in the hospital and joked around for over an hour. Larry told his parents he was glad when Caan left because he hurt so bad from laughing at his jokes.