Revealing mistake: Just after the F-35 explodes there is a shot of the flames receding up the ramp McClane slid down. Also in this shot, Rubble goes uphill, showing the shot was reversed. (01:51:20)
Continuity mistake: When talking to AV-81 Gabriel's microphone has a wire round it. The position of the wire changes instantly. (01:47:50)
Audio problem: When McClane and Ferrel are confronted at Woodlawn by the bad guy, the bad guy says "You, fix it." When he does, his mouth doesn't move for "You." (01:33:00)
Continuity mistake: Throughout the film, the injury to Farrell's ear constantly changes severity. (01:42:10)
Continuity mistake: Just after the explosion at the power centre, Farrell's hair changes from messy to neat when he says "Win? When did we start winning?" (01:14:20)
Continuity mistake: When McClane and Farrel are talking about what Farrel would do next if it was his fire sale, McClane and Farrel move closer together instantly. (00:54:00)
Continuity mistake: When John and Lucy are having an argument at the start of the film, Lucy's hair is over her shoulder or not over her shoulder, depending on the shot. (00:04:20)
Continuity mistake: When the 4x4 drops in the lift shaft, Ferrel is stood by the side of the door. Then as he says "Did you see that!?" he has changed position instantly. Then he's back to the position he was in before. (01:09:25)
Continuity mistake: When in the traffic jam, just before the first accidents, the background doesn't move in the shots of Ferral, but it does, and quite fast, in the shots of McClane. (00:22:00)
Continuity mistake: When McClane is strangling Mia, the way he holds the cable constantly changes. (01:08:05)
Other mistake: At the end Farrell is in the ambulance and talking to McClane. When doing so his hair is perfectly clean. After being through what he and McClane have been through in the last 24 hours, it's extremely unlikely his hair would be that clean. (01:58:35)
Continuity mistake: When he's talking to Ferral about a fire sale and what would be next, the cuts on McClane's face are constantly changing. (00:55:35)
Audio problem: When McClane is walking through Woodlawn and talking to Gabriel over the radio, his mouth doesn't match what is being said. (01:38:50)
Continuity mistake: When in the tunnel after blowing up the helicopter, John has got blood all over his face. However the amount of blood and the position of the blood keeps changing. (00:43:20)
Revealing mistake: In the tunnel, as John does the 180 handbrake turn, if you look on the floor you will see skid marks exactly where he performs the 180 turn. Clearly from previous takes. (00:40:00)
Audio problem: When McClane and Ferral arrive at Woodlawn, Ferral says something about the cooling towers being on the lower floors. As he does, his mouth doesn't move at all. Granted his mouth is blocked for most of it, but you can see a bit of his mouth at points and it's not moving. (01:30:15)
Audio problem: When at the Warlock's place, McClane says something along the lines of, "What're you guys talking about?" When he does, his lips are out of sync with the audio. (01:29:50)
Continuity mistake: When McClane tells Matt to get ready to run when in the apartment, McClane's position changes instantly. (00:14:05)
Revealing mistake: When the bomb explodes in Farrell's apartment, you can see the man in the room who gets blown against the wall is a dummy.
Other mistake: When the lights in the tunnel are switched off, you see that the cars that have been directed into the tunnel have their lights off. When you enter a tunnel, you are required by law to turn on your lights despite the lights in the tunnel. Sure, one or two drivers might ignore this obligation, but the whole avalanche of cars?
Answer: The radio line mentioned the officer's name in the seconds after Gabriel's voice recognition picked up Farrell's name.