Revealing mistake: When McClane goes into the icy room in the power station, look at the pipe to the side of the acrobat: it's in shot a few seconds and you can see that the writing on the pipe is reversed, revealing that in the editing room, the shot was flipped.
Revealing mistake: When the Sergeant has the shotguns in the backpack on his back, and you see a close up of his face, you can see that the shotguns do not have triggers. On plastic prop rifles, the trigger is usually molded into the trigger guard in the pulled back position, and you can see this on these prop shotguns.
Revealing mistake: When Conrad throws the Russian over the cliff, as the Russian explodes, you can see that poor CGI actually makes him transparent.
Revealing mistake: When the C130 is taxing on the runway (just ahead of the police search of the plane) you can see a C-17 in the background. The C-17 only made its maiden flight in 1991.
Revealing mistake: When Jack Gramm's Porsche explodes and make a flip in the air, the rear compartment where the engine is supposed to be is empty, and the rear axles and the tailpipes are common pipes.
Revealing mistake: When Art is hitting the Trevor (the blinded man) character with the hammer/hatchet in the beginning, if you look closely, you can see blood coming from Trevor's head, yet there is no wound or cut. The blood is actually coming from the prop that is hitting him in the head. (00:09:00)
Revealing mistake: When they blow up the whale, it is obvious it is just a large prop, as there isn't much blood or tissue flying through the air. You can tell by comparing it to the famous news reel online that shows what happened when the exact same thing happened in real life.
Revealing mistake: When the police dog wants to attack Swagger and bites the cop instead, it's obvious the cop wears thick protection under his coat sleeve. (01:03:20)
Revealing mistake: In the jazz club, the saxophone player's finger movements don't match what he is playing.
Revealing mistake: The '70 Challenger doesn't have a framed door - thus no place to anchor the belts.
Suggested correction: They're anchored to the floor.
Revealing mistake: When Kale and his father are hit by the red truck at the beginning of the movie, a rectangular hole can be seen in the trunk where the spare tire is, obviously not for the tyre itself or made by the crash. (00:04:25)
Revealing mistake: After "Do you want to die with dignity" all hitmen drop their magazines of blanks.
Revealing mistake: When Joseph goes to pick up the prisoner, there's a close-up of the paperwork; the prop is something recycled, or perhaps a real prison movement slip with Alex Veadov's picture stapled on, because the character's name is Vadim Nezhinski, a white Russian, but the documents say "Anderson, Robert", with a B (Black) for race. (01:09:45)
Revealing mistake: When the boy accidentally shot himself, the gun wasn't pointed in the right direction at all.