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.
During the final chase scene, Callahan and the bad guy are running down a wharf. In one view, Callahan is running through a column of steam. in the next cut, he is well beyond the steam, and the bad guy is running through it. Yet again, in the next cut, Callahan is exiting the column of steam.
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, in lieu of the fact that they pulled forward a few feet before the bomb exploded.
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.
After the killer attacks the cameraman in the garage, he gets in the news van and holds up a bloody knife to Samantha. In the next scene when they are in the shipyard building and he holds the same knife up to Samantha's throat, there is no blood on the knife.
In the final scene between Callahan and Rook, Callahan walks towards Rook holding a harpoon gun with a spool of rope, easily 25 feet in length, around his left arm. The next scene is a closeup of the harpoon gun, being aimed, but the rope is no longer on his arm. The next scene is of Callahan with the rope back around his arm. And then again a closeup of the harpoon gun with the rope off his arm.
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 to the inner yard, the film crew can be seen reflecting off the chrome bumper. The soon following shot of the two men getting out of the vehicle reveals a completely empty courtyard.
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.
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.
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.
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.
The goof items below may give away important plot points.
When Callahan leaves the killer's apartment, he takes Capt. Donnelly's car to where the killer is holding the news reporter. After Callahan finishes off the killer with a harpoon gun, Capt. Donnelly drives up in the car Callahan took.
Near the end of the movie, as the killer is walking towards where he will eventually die, we see him walking away from the camera with a bright light behind him and his shadow on the ground in front of him. To the right of this, the shadows of the camera man and other crew members are visible on the ground.
The C4 explosive composition the killer used typically came in 1.25 lb. blocks and measured 1" x 2" x 10". Although malleable to nearly any desired size, the radio controlled car used by the killer was an RC-10 radio controlled model of the 1963 Corvette, and it does not have any room inside of it to carry enough C4 to destroy a car, let alone kill anyone.