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)
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)
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)
Revealing mistake: John McClane stands in a little pool when the helicopter falling from the roof explodes outside and the windows shatter. In a short shot you see that this man is definitely not Bruce Willis. (01:52:50)
Revealing mistake: When John fights against the blonde German thug, Bruce Willis' stunt and the thug's are very noticeable: different body build plus darker and longer hair in the case of Willis, and bulky wig in the case of the German. (00:35:21)
Revealing mistake: Even though there is no light shining on John McClane during the fight with Karl, there are several shots where it's obviously not Bruce Willis, but a stunt double.
Revealing mistake: After escaping from the roof, John walks inside the building. When he runs towards a huge vent his bare feet swap to wearing black shoes.
Revealing mistake: After barefoot John rolls down the stairs with the German thug one can notice the top of John's (his stunt in this case) protective sock around his left foot. (00:35:44)
Revealing mistake: Having leapt from the rooftop, Bruce has his feet planted on the window. As he then goes to push himself back in order to smash the glass with his gun shots watch the rope. Someone above has kindly held the rope out several meters away from the building in the next shot, allowing him to get a much bigger swing.
Revealing mistake: Bruce Willis jumps off the roof with the firehose. The hose hanger breaks off the wall and apparently gets stuck in the railings. The shot is very careless: slow motion shows that the firehose doesn't get stuck, but simply clings like a magnet to the top of the railings. (01:51:50)
Revealing mistake: When McClane is on the roof shooting over the hostages the helicopter appears and they start shooting at McClane, the squibs used for the helicopter shooting scene are visible lying on the roof and explode in sequence, starting at the back and moving toward the camera as McClane does.
Revealing mistake: When John McClane jumps from the explosion in the elevator shaft, you can see that it is not Bruce Willis but his stunt double.
Revealing mistake: As Bruce Willis walks into Nakatomi Tower, which is solely occupied by the Nakatomi corporation, behind him can be seen the logo of Merryl Lynch, which had offices in the building used for filming.
Revealing mistake: The first time John walks up to the rooftop, he walks backwards to avoid the thugs' bullets and shelters behind a wall. Despite supposedly being barefoot his socks are very noticeable and wrinkle around the ankle.
Revealing mistake: When John jumps off the roof with the hose tied around him, you can see that the stuntman's hair is slicked back with fireproof gel during the explosion.
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.