Factual error: There are a lot of comments about the Harrier (technically speaking it is an AV-8B, a slightly different aircraft) flown by Arnold Schwarzenegger, but everyone's missed the obvious one. The terrorist with the AK47 is unaware of the fact that a Harrier is hovering just outside the window until he slowly turns and see it there. He does a double take when he sees the Harrier just five metres away! A Harrier is an incredibly loud aircraft; you can hear them from three kilometers away. At that range the floor would shake with the noise and he would have been in no doubt whatever that he had been under attack for some time.
Deliberate mistake: In the scene where the bad guy is attached to the missile and fired, in the split second before it hits the helicopter you can see below it a building called SUN BANK, but the letters are backwards. A close look reveals the likely reason - to the right of the reversed building is one which looks just like the building to the LEFT of the Sun Bank building behind the harrier a few seconds earlier. Most likely they couldn't film the chopper from the right side because there was a building there (the one the camera flies through), so they filmed it from the other side (the same side they'd filmed the harrier from), catching the "Sun Bank" behind, then had to flip it to have everything facing the right way. (02:06:55)
Revealing mistake: During the shoot-out in the restroom, the villain fires shots down the length of stalls. Then as he walks down kicking each door open there is a shot from inside one of the stalls. You can see where the explosives were taped to the back of the door to make the gun shoot holes. There is residue on the back of the door and all of these rectangular clean area where the holes ended up being. (00:31:50)
Revealing mistake: During the scene where Arnold's character is escaping from the mansion at the beginning of the movie, there is a part where he is running down a snowy hill through trees. At one point during the hill scene, Arnold does a flip head first. When he lands the flip, you can clearly see the stunt double's face as he turns to shoot at more bad guys. (00:13:30)
Revealing mistake: After the bathroom shootout, when Harry Tasker has his team contain the terrorist leader in front of the building they're about to chase through, just before the terrorist rides the motorcycle through the front door, if you watch carefully, you can very clearly spot Arnold Schwarzenegger's double on the horse. (00:35:35 - 00:36:40)
Continuity mistake: In the scene when Harry brings Helen's purse to Gibbs in the van, it is raining very hard. When Harry is standing along side of the van talking to Gibbs who is in the van, he is not getting wet and there is no rain falling when the camera is facing in towards the van. However, when the camera angle is facing out away from the van, it is pouring rain and Harry is getting wet. (00:49:15)
Continuity mistake: In the scene where Harry is trying to get the truth from Gibb about Helen's phone conversation, the van pulls over and the wheels are facing to the right. They get out and talk and Harry then punches the window through. As they get back in the van and pull away, the wheels are facing the opposite direction. (00:56:55)
Factual error: At the start, when Schwarzenegger leaves the party to steal the computer evidence, when he turns on the computer you can clearly hear the Macintosh start up sound, but when he looks at the computer screen it is the Windows loading screen. It's not a case of changing the sound files - PCs make no noise before Windows is fully loaded except perhaps the initial beep. (00:07:15)
Factual error: Throughout the entire movie, Arnold and Tom are always riding in the GM S-series SUV. The thing about these vehicles is that they came with different side view mirrors and if you look close enough during all the shots with the SUV, the side mirrors on the side change, from small ones to big square ones, showing that two different GM S-series SUVs were used throughout the movie.
Continuity mistake: The scene in the SUV where Harry asks Tom Arnold 'if there is anything new', and he answers 'no', you see that they were driving straight, but when the camera does a wide shot, you would see a building directly behind them which means they would have been driving through the building.
Revealing mistake: When all the terrorists are firing their guns at the sky near the end, one of the men is clearly pulling the trigger, and empty cartridge cases are coming out, but there is no muzzle flash like the other guns.