Deliberate mistake: For Hans Gruber's iconic death scene, Alan Rickman's fall was filmed at high speed (for slow-motion playback) against a green screen, and the skyscraper perspective footage was added later as background. However, while Rickman falls away from the camera in slow motion, papers are fluttering around him in the background at normal speed. This was done deliberately to make the shot even more surreal.
Continuity mistake: After he climbs out of the air duct, John McClane's shirt goes from white to green. The dirt and grime from the air duct would cause discolouration only where the shirt comes in contact with the duct. However, his shirt is uniformly green including places that wouldn't make contact and it is impossible for all surfaces to make contact even in small areas. (00:23:05 - 00:56:30)
Other mistake: Powell says he has a desk job, but he is driving a cruiser. Cruisers are for cops on patrol. A cruiser would not be issued for a cop at a desk, especially not one who's on his way home, as Al says he is.
Visible crew/equipment: In the first scene of the Christmas party when Takagi steps out of one the offices and greets a couple of his guests, the camera pans left to show the orchestra and other guests. A huge shadow of the camera and its operator is being shown on the back wall, then the stage light gets moved to light a different area and the shadow is gone. (00:02:20)
Continuity mistake: When the terrorists launch the rocket at the RV, they break the same window of the building twice. (01:11:55)
Continuity mistake: When Hans takes Holly hostage, as she is sitting on the floor talking to Hans, as the camera goes from him to her, each time Holly's shirt is unbuttoned and open more so her bra and boobs are revealed.
Revealing mistake: When Hans discovers the first bad guy lying dead in the lift ("Now I have a gun - HO HO HO!") and pushes his head, the corpse is moving his head before he is hit (watch carefully). He also closes his eyes before Hans makes contact. (00:39:15)
Other mistake: When Bruce is looking on the touch-screen computer to find out where his wife is, he touches the name GENNARO, but when it changes colour it also corrects the spelling of his wife's name to GENNERO. (00:09:25)
Other mistake: In the shot where we see all the terrorists (except Karl and Theo, who are upstairs) walking into the building from downstairs, they are about to walk through a doorway when the camera cuts out. Watch the terrorist on the left (he's the one who guards the door) - the way he's walking he's going to smack straight into the wall! The shot cuts a fraction of a second before he does, but that's not a remotely normal walk/position - it's clearly just because it makes for a cool looking shot. (00:18:15)
Revealing mistake: Bruce Willis wore fake feet during the movie to protect his own while shooting the film. They were like boots that slipped on over his. At the part where the FBI guy shoots at him from the helicopter and Bruce jumps to a lower level of the roof, you can see the tops of the fake feet that aren't tight around his ankles. (01:50:30)
Factual error: The cops try to get in the building by cutting some metal part of the door, it might be the lock. All they have to do is break the glass. SWAT teams aren't going to worry about a bit of property damage with hostages' lives on the line.
Factual error: Throughout the film, characters are able to break through on radio transmissions and speak to the other person talking, and the other person would hear them. In reality, they would have to wait until the first transmitter un-keyed the radio before any other discernable radio traffic could have be heard. (A good example of this is the scene where Ellis is giving up McClane.) (01:18:40)
Visible crew/equipment: When one of the terrorists enters an abandoned floor, logically the floor should be empty, but you can see the casting crew in the reflection of the windows. (00:35:00)
Continuity mistake: When the terrorists enter the building, they stream out of the rental truck. You don't see the large ambulance that emerges for their getaway at the end of the film. This is because the ambulance was a late addition to the script - the terrorists leaving the truck was originally a longer shot, but had to be cut down as short as possible to hide the lack of ambulance. More info in the trivia section. (00:18:00)
Other mistake: After Powell frantically radios in his distress call, other police cars converge on the street in front of Nakatomi Plaza, blocking off both sides of the street. But when McClane grapples down the building using the firehose, you can see traffic on the street below and none of them are police.
Continuity mistake: When Bruce Willis shoots Alan Rickman, Rickman falls through the window and grabs Bonnie Bedelia's arm to pull her with him. He has a good grasp on her arm because of her watch. When Bruce Willis is trying to release the clasp on her watch, you see blood on her face from his arm. The shot then cuts to Alan Rickman, and when the camera goes back to Bedelia and Willis, there is no longer any blood on her face. (01:57:15)
Other mistake: Near the start of the film, when John goes to meet his wife at the office block, there is a party going on and a small orchestra playing. When Gruber and his gang enter, the orchestra are still there. Gruber locks down the building, and takes the guests hostage. Where does the orchestra go after that? They are never seen again, even when the hostages are taken to the roof.
Revealing mistake: When the hostages are running down the stairs after the C4 explosion, a large, fist sized chunk of concrete falls on an older man's head and he just runs straight on. (01:52:35)
Revealing mistake: When John and the terrorist fall down the stairs together, you can hear the snap of the terrorist's neck signifying his death but at the bottom you can see the terrorist's "corpse" is still breathing. (00:36:00)
Other mistake: In the computer room shoot-out the bad guys shoot out all the glass room dividers in a hail of bullets, but the glass windows on the exterior of the building (behind the dividers and in the path of the bullets) stay intact; they don't even get scratched.
Answer: We don't know what John would have done in that circumstance. Obviously Hans was planning to kill everyone with the explosives anyway at the end. Perhaps John would have suspected that. Also, doing that would invite more police incursions.
Greg Dwyer
Hans thought Ellis was a good friend of John's and John still didn't give up when he was going to shoot him. If John wouldn't save his friend, why would he care about others. Plus Hans told Karl earlier he could stall the police but not if they heard gun shots. The police would have absolutely stormed the building if he started killing the hostages.
Zorz
The fact that we don't know how John McClane would have acted doesn't remove the fact that it would most likely have been a good way to coax him out. Also, depending on when Hans Gruber would have decided do implement this strategy, John probably wouldn't have known about the explosives on the roof as he only finds out about them at the 3rd act break. As for the "more police incursions" part, I couldn't disagree more; Hans already killed two hostages - one on speaker with the police -, all the cops in LA seem to be there already, and don't forget that the involvement of the FBI is part of their plan anyway. This is definitely the one major plot hole of this otherwise perfect film.
It would have been, but plenty of movie plots don't pan out the "perfect" way without it being a plot hole. Killing Ellis is a reasonable first step, it doesn't work, and then the events of the plot pick up pace - Gruber goes to check the detonators, as that's a priority. He's hoping/assuming they can get through the rest of their plan by isolating McClane, or at least prevent him causing more chaos. They want the power shut off - they don't want to cause such massive carnage that the building is stormed before then. They need to get helicopters, blow the roof, and escape as planned. Hans doesn't want to derail things any more than they already have been.
Jon Sandys ★
Seems to me like they have all their bases covered; the police isn't even able to get in with a tank as he blows them up so I don't think the police "storming the building" is even a possibility in the reality of the film. Also, after blowing up that tank, that's two hostages and a bunch of cops dead so I would say the situation is pretty derailed. Everything is going as planned for Hans and his team, except for McClane, so he should be in damage control mode and this is an obvious solution. He doesn't even have to change his plans, just tell McClane he's gonna kill one hostage every 10 minutes until he shows up unarmed and tell one of his henchmen guarding the hostages to do it while they go along with the plan and maybe even try to find McClane at the same time. I think this is something Hans should have at least considered, but the screenwriters just didn't think about it/didn't want to address because they couldn't think of a good reason for him not to do it.
There are no cops dead, Hans says "Just wound them" and despite the awesome explosion, the APC isn't actually penetrated or destroyed. But Hans needed this to turn into a standoff, a show of force would prevent a SWAT raid from expediting the deadline, he needed to get all of the hostages up on the roof to make his getaway downstairs, and executing a bunch of them would bring suspicion onto how cooperative he is (His plan to blow up the roof relies heavily on the police sending in choppers) they cooperate with him, which they won't do if they think Hans is a crazed lunatic who's only interested in more and more carnage, if he wounds the cops and only shows he can defend himself, and that he was being reasonable. The cops would play ball, and they would believe he's willing to spare the hostages lives, plus he always planned on taking one hostage as a contingency, if they thought they were gonna be killed they'd become a liability. Patton Oswalt talks of a real plothole though lol.
John McClane would know they'd kill him as soon as he shows up, as soon as he heard "We'll have to tell Karl that his brother is dead" he knew that all bets were off, he lost his chance to end it civil, if they had no personal connection to the first terrorist John kills then maybe putting 5 people into a room and doing an Air Force One on them would work, but not when John knows he'll be body number 6. Al says it best "If he gave himself up they'd both be dead" with Ellis execution, John watched them take control of the hostages, watched them execute the Takagi, and when the first Terrorist thinks he's found John he shoots first after saying "I promise I won't hurt you" and then taking his bag and realizing how well financed and equipped, these guys weren't domestic terrorists, they used serious money, serious contacts, and serious planning to get themselves into this building on this night. He knew the only way to play ball with them was fists and elbows.
Just because a character doesn't do a thing I doesn't make it a plot hole. The plot was that he didn't do it. You may consider a different approach "better" but that's irrelevant. You may as well try to argue that any character choice that doesn't fit with a perceived meta is a plot hole. It isn't, it's just the plot.