When Harry and Al are investigating the death of Johnny Squares in his trailer, Squares has a rubber strap around his left bicep. A moment later as Callahan lifts his arm to describe the tract marks on his arms, the strap is gone.
The first car bomb with the remote control toy leaves no trace of its victim, yet when the next car bomb detonates under Callahan and his partner's vehicle, they are able to walk away with minor injuries, because of the fact that they pulled forward a few feet before the bomb exploded.
When Harry (after firing one shot) drops his Model 29 at Rook's demand, he discards it about two feet to his right. When he kicks it over to Rook, however, the gun is now lying in front of him, atop the steam grate.
Liam Neesons character has a pony tail - a little hair extension clipped to his real hair. After the funeral scene, Peter Swan (Neeson) is walking towards the camera with Harry and turns his back to the camera. He has a small amount of his hair in a pony tail without the hair extension. In the next shot with a side profile, he has his pony tail extension back in place.
In the remote control car chase scene, when the toy car goes under Harry's car, the taillights are an Oldsmobile Delta 88. During the rest of the scene, Harry is driving an Oldsmobile Ninety Eight.
In an other fallacious furthering of 'movie physics', the "suicide guy" is not instantly immolated by holding not one, but two lit road flares after having "soaked himself with gasoline", and then pouring even more of it on his pants. There seems to be no risk of combustion until the liquid gas reaches the discarded flare on the ground, despite 1) the flare constantly shedding sparks into the pool of gas on the ground, and 2) the seemingly unknown fact that gasoline in it's liquid form is not flammable. It is the fumes from spilled gas that will ignite, and that would've happened long before Callahan et. al. arrived on the scene.
The tiny RC-Corvette would've dropped into the open manhole, not flown over it, since there was no ramp for the car to jump from.
Several of the dead persons are seen being taken by EMTs to an Ambulance. In California it is forbidden by law for an ambulance to transport any dead persons.
After the restaurant shootout Detective Quan is seen going through one of the shooting victims he finds "The Dead Pool" paper. In California is if unlawful for anyone to touch the decedent without the express permission of the Coroner or Medical Examiner.
During the opening scene when Harry is ambushed in his car he's shot at by all the people, wrecks the car, then he gets out and kills them all. The next scene shows a tow truck pulling away Harry's car, and you can see bullet holes through the driver side door and driver side rear door in reality if this happened (even with low calibre ammo, which was apparent they were not because of the weapons used) bullets will easily penetrate car doors most times go through one side and out the other (if not hitting what's inside first, i.e. like a person) so if the car were shot where the bullet holes were at, Harry would have surely been killed.
When Johnny Squares is lip syncing to "Welcome to the Jungle", he's clearly getting many of the words wrong. Johnny is withdrawing from drug use and is affecting his ability to sing in time with the music.
When Harry hands over the gun to Rook, the gun has already 6 bullets, but in the end when Harry tells him "You're out of bullets", Rook has fired only 5 bullets.
Possible explanations:
(1) Harry might have already fired one shot in a previous scene; we don't know how many times he reloaded.
(2) In the stressful flurry of just trying to keep himself and the lady alive, Harry might have miscounted Rook's shots ("I know what you're thinking --- did he fire six shots, or only five? Well, to tell the truth, in all the excitement, I've kind of lost track myself.").
(3) Harry might have lied to cause Rook not to shoot at him that last time.
(4) Harry might have only loaded five rounds the last time he'd re-filled the cylinder.
Note that before Harry hands the gun over, he fires one shot at Rook's boom-box.
After the killer attacks the cameraman in the garage, he gets in the news van and holds a bloody knife to Samantha. In the next scene when they are in the shipyard building and he holds (presumably) the same knife up to Samantha's throat, there is no blood on the knife, but he may have cleaned it, or it may be a different knife.
During the toy car "car chase" scene, it is clearly seen that both Harry's and the killer's cars have extensive skid plating (each covering virtually the entire undercarriage) when they fly through the air. No street car would have that, and even off-road vehicles built for rocky terrain don't have skid plates cover anywhere near that much undercarriage.
At 34:26 When Harry first visits the news room and Samantha is saying goodnight the teleprompter next to him says goodnight and good riddance.
During the chase with the remote controlled car, after Callahan's car makes it's second jump over a steep hill, you can briefly see a portable film camera on a mount in the back seat of the car.
At the end of the funeral scene, just before Clint walks off camera, the large headstone behind him (Allan Pache 1960-1970) is clearly one of the fakes as it doesn't go into the ground. It seems to be hovering slightly, obviously just placed over the top of one of the real stones.
Around the 64 minute mark when a car is follow the jogging Callahan through the gateway and stopping in the parking lot, the film crew can be seen in the reflection of the chrome bumper. The following shot of the two men getting out of the vehicle reveals a completely empty courtyard.
Following the opening scene shoot out, the camera man's shadow appears on the car as they tow Callahan's bullet riddled car away,
Director Peter Swan tells Inspector Callahan that he encountered Harlan Rook at every film screening or other public appearance, where Rook's comments showed that he knew everything about Swan's films. Rook also constantly pitched his screenplay to him. Despite this extensive contact and familiarity, Swan does not recognize Rook when he works closely with him on a daily basis as his special effects supervisor under a fake name.
During the remote control chase sequence the killer is shown using a twin joystick remote control unit, leaning on the dashboard, to drive the RC vehicle. At the same time Rook is driving his own car in a high-speed pursuit. Given that it takes both hands to use this kind of remote he cannot do both simultaneously. He also clearly loses sight of the RC vehicle at various times yet manages to keep it driving perfectly straight. The concentration and dexterity required to simultaneously drive two vehicles with totally different control methods in a high-speed chase through busy, twisting streets whilst unable to see the remote vehicle is so astronomically high as to make it impossible.
When the machine-gun goons are shooting at the couple in the glass elevator, they continue to aim for the waist-high and above windows, even though they plainly saw their targets drop to the floor of the otherwise-unprotected elevator car.
Callahan and Quan as members of the San Francisco Police Dept. are being chased through the streets by a madman with a radio controlled car bomb, and they don't radio for help? They don't call for backup, relay their position, they don't have portable police radios or even use the supposed police radio in their car. The SFPD would have been all over that chase long before it ended.
Quan was said to be a Chinese-American. However, when Callahan noted the markings on Quan's arm, the markings are in Korean: Kim Jong Seo.