Continuity mistake: When they run onto the roof near the start, there's another trooper behind Harry running onto the roof (only visible in widescreen - in 4:3 it looks like a weird reflection). In the following shot of Keanu Reeves you can see the reflections of Harry and the other guy on the plastic covering the scaffolding. We're led to believe that Harry and Jack should be the only ones up there, as everyone else has a job to do. Aside from those two shots, this third trooper disappears once they start hooking the crane up. (00:11:30)
Continuity mistake: When Jack and Harry are on the top of the building at the start, Jack climbs up a ladder and proceeds to walk along to the crane. In one shot he moves the big gun round to his back, and in the following shot the big gun is still in front of him. (00:11:55)
Other mistake: As the first bus explodes on its way from the cafe, we can see a blonde pedestrian casually walk past a window with her ears covered. At no point does she even stop to glance at the burning wreck. It seems (from her perspective) as though the explosion were merely an inconvenient loud noise and nothing worth noticing. (00:27:55)
Visible crew/equipment: On the train, Jack hears Mac saying the track isn't finished, then Jack becomes aware of the broken emergency stop button. When Jack walks back over to Annie and tells her they have to jump, we can see the reflection of the standing camera operator on the right door's window, and the reflection of a crouching crew member (possibly boom operator) on the left door's window. Definitely crew given they're meant to be alone on the train. (01:45:55)
Continuity mistake: At the beginning, when the employees are getting into the elevator, there is a close view of the man at the front pushing the button with his finger, but when it cuts to a wider shot, he is taking his thumb away from the button, instead of his finger. (00:04:35)
Continuity mistake: After Jack shoots Harry in the leg, Denis Hopper runs out the door which is wide open. The explosion shot (seconds later) happens and we see the door completely shut. (00:22:05)
Visible crew/equipment: Close to the beginning of the film we see Jack in a coffee shop and his friend walks in. When they both leave and Jack goes to his car, a cameraman is reflected in the car window after the palm tree. (00:26:40)
Continuity mistake: At the beginning of the movie after Jack has hooked up the crane from outside to the lift, there is a shot of the crane outside, and the line going through the vent. If you look closely there is a pipe in front of the vent, yet when the crane goes through the vent, the pipe has vanished. (00:14:20)
Continuity mistake: When Jack handcuffs Ray to the metal handrail, Ray's right wrist has the double strand bars facing up and single strand bar facing down, but when Jack tells the passengers about the gap and then uncuffs Ray before the jump, the handcuff's single strand bar is facing up on his right wrist.
Revealing mistake: The Jaguar owner holds up the piece of paper with "bomb on bus" written on it, then he loses his grip on it, and when the paper lands directly on the bus windshield, the stunt wires are visible at its sides. (00:37:35)
Revealing mistake: When Harry and Jack arrive at the building at the start in the police car, you can see the tire skidmarks on the road from previous takes. (00:05:05)
Continuity mistake: In the cafe, Jack shouts to Bob "Have a good one" and holds his arm out with his coffee in his hand. When the shot changes and Jack is turning back to the counter, his coffee has swapped hands. (00:26:20)
Continuity mistake: As McMahon watches Harry stumble out of the bar, he takes a puff of his cigar. Between frames, his right hand quickly moves from the cigar to the counter-top. (00:26:30)
Continuity mistake: When the crane breaks free from its bolts and starts flying towards some ventilation windows, it obliterates the upper pair. However in the next view from within an air duct, both of these windows just swing open freely as the crane busts through, and yet they barely appear to have suffered a scratch or bend in the process. (00:15:00)
Continuity mistake: When the crane is torn out of the floor and flies towards the ventilation window, from a shot behind, the holes on the base (where the screws used to be) are gone. (00:14:30)
Answer: He's taller now because he has his head while the villain does not!
Grumpy Scot