Factual error: In the tower, Leslie Barnes and a few men are talking about the Annex Skywalk, mentioning that it is a "VHF System." Barnes says the planes are so close it won't matter. VHF, or Very High Frequency, transmits far distances, hence why it is used by airports around the world. He should have said "UHF", which transmits shorter distances, as that would make more sense.
Continuity mistake: In the airport lobby, when the two policemen come at the bar they have "snow" all over their clothes. In the next shot they are shown from behind sitting at the bar and Lorenzo's brother has no snow on his jacket. Seconds after Mc Clane comes to talk to them and the cop has snow on his shoulders again.
Factual error: A dangerous prisoner like Esperanza would not be flown into a civilian airport. He would be transported to the nearest USAF base, in this case Andrews Air Force base, always assuming that he is being extradited to Washington D.C. As well as this his wrists would be shackled to a chain around his waist which would also be c attached to ankle shackles. He would also be escorted by half a dozen armed police officers.
Continuity mistake: When McClane grabs Marvin the janitor and pushes him against the lockers, McClane's thumb changes position on the gun, some shots it's on the hammer, other shots it's around the handle.
Revealing mistake: McClane jumps out of the helicopter at the end and lands on the wing of the larger airplane. The stunt man who does this then proceeds to move up to a long wire on the side of the wing. Any normal airplane would not have a long wire on the wing.
Audio problem: John is on the wing of the plane, questioning why he is there. His mouth isn't moving with what he is saying.
Continuity mistake: When John and his wife hug, his hand suddenly appears on her head.
Continuity mistake: When Barnes and the SWAT team reach the Annex Skywalk, they get on the moving sidewalk. Before the terrorist stops the moving sidewalk by pressing the button, the sidewalk is already stopped, even though the light was still green at the time.
Continuity mistake: When the general exits the plane with Colonel Stuart, nobody shuts the door. It is closed in the very next shot.
Answer: If you listen, he says something to the effect of "I know I had that guy in my sights", meaning he is amazed he missed. So he checks the bullets in the magazine to see if that's the problems. Blanks are simply shells with no bullet that are crimped shut to hold the powder in. The difference between blanks and live rounds is very obvious even at a glance.
Grumpy Scot