Other mistake: After the scene where the building explodes when the gas was rerouted to the station, there will be a shot of the main bad guy staring at the computer screen saying the connection failed. On one of the two connection lost screens, it says, "connection faild", this is not how you spell "failed". (01:16:45)
Factual error: There are no toll booths, tunnel or otherwise in the area of Washington D.C. where the anti-helicopter car launch is supposed to have taken place.
Audio problem: After the terrorist in the helicopter opens fire on John and Matthew, a policeman yells something about backup into his radio. What he's saying doesn't match up with his mouth. (00:37:45)
Continuity mistake: During the final truck chase through Baltimore, several traffic lights are seen with lights in them. But all the power was off.
Factual error: When Gabriel turns off the lights in the tunnel, we see a shot of cars heading towards the police car as the lights turn off, and not one of them turns its headlights on. In reality, with that many cars in the tunnel, there'd be at least one that was fitted with auto-lights.
Factual error: Just after Bruce Willis slides down the ramp, and gets up to walk away, there is a car that supposedly fell off the highway. This car is turned upside down. It is obvious that the transmission and drive shaft have been removed from the vehicle, making it undrivable. (01:56:15)
Continuity mistake: In the scene where McClane rebuffs Ferrell for thinking it would be cool to do the firesale, you can see pipes for firehoses right behind McClane. When the shot changes to a long alley shot, the pipes are a good ten feet away, but when it reverts back to a closeup, the pipes are back to right behind McClane. (00:55:20)
Factual error: If the natural gas could be rerouted to McClane's position at the national grids control center, there are two major problems. The gas would not self-ignite on its way there as pictured, and the sheer momentum of that massive amount of gas in those long pipes would cause a massive leak at the endpoint. However, after the first (relatively small considering the total energy in all the gas) explosion, everything goes back to normal without one giant amount of gas erupting.
Factual error: In several chase scenes that take place in Washington DC McLane and Farrell are driving around the city with buildings that are at least 40 to 50 stories tall. This is impossible as the Height of Buildings Act of 1910 prevents any building being any higher than 20ft more than the width of the street the said building sits on. The buildings seen in the chases are much too tall for this city to actually be Washington DC.
Continuity mistake: At the start of the chase between the HAZ-MAT van and the semi truck, McClane hits a couple of cars and seriously damages the front drivers corner of the truck. In later shots, the damage has gone. (01:48:55 - 01:51:50)
Plot hole: The hackers cause a massive traffic jam by manipulating the street lights. Yet, only minutes later, the streets are conveniently empty when McClane is being chased by the chopper. Sure, one or two streets were secured by police, but the chase scene goes through many more streets. (00:23:15 - 00:38:55)
Continuity mistake: During the shootout at Matt's apartment John scrambles away from the refrigerator and pulls himself around the corner. The view is from behind John and his chest is facing the corner as he pulls himself around it. The scene jumps to a view from in front of John, yet he's somehow already turned himself around so his back is to the corner (this makes for an easier transition to using the corner as cover, but it messes up the continuity). He couldn't have done this since he was still in the process of pulling himself around the corner.
Continuity mistake: When the truck crashes into the pillar that McClane and Matt are hiding behind in the tunnel, we see a large portion of the pillar fall off from the left side. However, when McClane gets up to drive the police car, the pillar is in perfect shape. (00:41:30)
Continuity mistake: In the scene after the power plant explodes and Farrell is giving his "What is the point?" speech, his hair changes several times between shots, from hanging down in front of his face to being swept to one side.
Visible crew/equipment: In the scene where John and Matt are in the middle of an alley, with Matt using the mobile to locate the power hub, the crew can be seen from the reflection from the glass behind Matt.
Continuity mistake: When McClane knocks the fridge over so it's on its side near the start, his head is above the top (side) of the fridge. Camera cuts and it is well below the top (side) of the fridge. (00:12:45)
Continuity mistake: During the traffic jam scene, when the truck crashes against the taxi, the view inside McClane's car shows the taxi colliding against the car, dragging it away violently. From the aerial angle, the taxi doesn't touch McClane's car at all. (00:22:35)
Factual error: In the beginning of the movie after the scene where John watches his daughter go inside, he is called on his police radio. The gentleman on the other end is in fact talking to him on a land-line telephone and not a radio, but from Johns end you hear the mic being keyed on and off. Police radios cannot make or place telephone calls and vice-versa. (00:06:45 - 00:07:50)
Suggested correction: Although very unpopular and very unlikely to have been installed in a Police car, some radios can indeed ring landline telephones. They were known as "Mobile Radio Telephones" but they tended to be extremely unreliable.
Factual error: Just before the power plant explodes we are shown a scene where the natural gas pipeline is exploding on it's way to the power plant. The first explosion (while not actually possible) would have destroyed the pipeline and no more gas would have flowed to the power plant and therefore the power plant could not explode.
Continuity mistake: In the traffic jam in the beginning of the film (right after all the lights have changed to green), the car next to McClane's is black/dark. But a few seconds later when you see it from a helicopter, it has changed to white. (00:22:30 - 00:23:30)
