Just after Harry gets word there's trouble in the airport, we hear a female voice over the public address system: "American Airlines Flight 557, Electra II Flagship, nonstop for Washington, now boarding at Gate 5." American Airlines stopped using Electra IIs in 1969. In fact, the full-size Lockheed Electra II turboprop had virtually disappeared from all U.S. airlines by the time of "Magnum Force."
When Harry is killing the hijackers on the plane, he has left his .44 magnum behind to disguise himself as a pilot, and is using a smaller revolver, probably a .38, but when he shoots, the gun makes the loud characteristic .44 magnum report.
In the 2001 DVD version of this film, the audio has been re-designed, and the resulting gunshots from the .38 sound different from Callahan's .44 Magnum.
After Harry discovers that there's a bomb in his mailbox, he goes upstairs to his room and pulls out his Smith & Wesson. He then puts it on top of his chest of drawers and searches for a screwdriver. Finding the screwdriver, he picks up his gun to leave, and puts his thumb on the hammer. The sound of a Model 29 uncocking is heard, but Harry doesn't actually uncock it because it was never cocked.
As the motorcycle cop is getting ready to attack the pool party, he is seen loading and cocking the submachine gun - the sound is a click-clack of racking the bolt back and forth but the weapon, a Smith & Wesson M76, fires from and open bolt (meaning there'd only be one click, the sound of pulling the bolt back into firing position). As a side note, the blank rounds in the magazine are visible for a brief instant as it's inserted in the gun.
When Harry walks up to the limousine in which Ricca and his men have been murdered, he puts his hands on the car's door sill and partially rolled-down window, contaminating an active crime scene which is still under investigation. No veteran homicide detective would do this.
Walter, the CSI, tells Harry, when he is looking at the pimp's car, that the shooter "emptied the entire magazine" into the body. However, in the previous scene Walter has already identified the bullets as coming from a 357 Magnum. The 357 is a revolver and therefore does not use magazines.
In the opening scene of Ricca leaving the courthouse, just after the man yells at the TV Reporter "Fuck the courts!" there is a quick crowd shot before a shot of Ricca trying to muscle his way to his car. In that quick moment, there someone holding a sign that is meant to say "UP YOURS RICCA" but contains a grammatical error as it actually reads "UP YOUR RICCA".
When Callahan jumps onto the hood of Palancio's car after the police raid, he is facing the rear of the car, grabbing the hood near the windshield. A few seconds later, he is seen sprawled across the hood, facing the front of the car, and then seconds later he is back in his original position.
When the pimp is driving across the Golden Gate bridge, he looks into his rear view mirror to check out the motorcycle cop following him. Cars that appear around the cop in the mirror are different than the cars around the cop in the non-mirror shot. (A small, red foreign car is visible in the mirror. When the shot immediately returns to normal, the red car is not there).
During the hijacking sequence, Insp. Callahan is eating a hamburger
at San Francisco International airport. When he walks out of the terminal to board the hijacked plane, he's at the Oakland International Airport across the bay.
When the prostitute is picked up by the cab, the exterior shots of the car are of 1963 Chevrolet. When the interior shots are seen, the car is actually a 1972 Ford with the seat from the 63 Chevy installed that are of the same interior color.
Near the end of the movie when the back windshield is shot out, the bullet passes close to Callahan in the front seat. Yet later when Briggs is driving away, the bullet hole is close to the passenger side of the car.
When Davis exits the parking garage after killing Charlie McCoy and confronts the curious pedestrians, he and the group are silhouetted by the dark shadow of the building that extends almost to the opposite side of the street. In the next shot, a reverse angle when Davis removes his helmet, he and the crowd are in sunshine and well lit.
At the marksmanship contest pistol range, when both Frank and Harry take their turns shooting at the targets, the spectator booth in the BG is in shadow. When cut aways of Early, his wife and Lt. Briggs in the booth are shown, they are lit much brighter than the previous shots with Frank and Harry.
In Harry's car, when Briggs says, "Let me have your gun," they're on a two-lane city street. When Harry hands it over, a few seconds later, they're on a four- or six-lane divided highway. In the next shot they're back on a city street. After Briggs says, "Now the shells," they seem to be entering that same divided highway. But the next shot is on another city street. (Incidentally, the reason we see a Union 76 gas-station sign appear twice behind Harry is that the same reaction shot of him is being used twice.)
Before the famous line, "Man's got to know his limitations", (very often erroneously quoted with a leading "A", as in "A man's got........", the camera angle on Harry's face changes drastically between just prior to the car exploding, and just a few seconds later when the cut goes back to him, as can be evidenced by the lettering on the sign behind Harry's face.
After confronting Callahan in the police parking garage, the motorcycle cops all start their motorcycles by hand, using electric starters. Yet, at the aircraft carrier late in the film, both motorcycles have to be started by foot, with kick-starters.
It's clear that where the scene of Ricca (the mob boss) being killed is not the same place where Callahan arrives at later to view it. The area where the shooting was shown was clearly partially-deserted. The later scene is a busy street with both pedestrians and homes close by.
In the scene where the vigilante cops come to arrest the mob boss at the warehouse, one of the cops stands to the right side of the door facing sideways and knocks. When the mob boss fires the shotgun through the door, the cop is standing directly in front of and facing the door and gets hit square in the chest.
The Sovereign Airways plane when moving down the runway with the front-on shot, has one engine under each wing, when the plane comes to a halt, after "Captain" Harry applies the brakes, there is another partial front-on shot of the right-hand wing, and there are two engines quite visible under the wing!
When motorcycle stops behind pimp's car, he and his motorcycle are partially in shadow, yet moments later the cop and his motorcycle are in complete sunshine. Bridge shadow is also inconsistent in earlier and subsequent shots.
After Callahan removes the bomb from his own mailbox, he looks up at a man on the nearby stairs. As a car drives by on the street, the lights from it cast the shadow of the boom operator onto the wall.
While Harry and his partner are following Palancio (and ask for directions to San Quentin), the four lights mounted on their vehicle are visible in Harry's sunglasses. Also the shadow of the camera mounted on the side of their vehicle repeatedly comes in and out of view as the car turns.
During the shootout at Palancio's hideout, a gangmember attempts to escape in a Ford sedan - Harry fires a shot at the driver and the car crashes into the office building/hideout. Just before the car crashes through the wall, a large object can be seen in the back seat, covered with a black cloth, which is the camera for the POV shot as the car smashes through the wall.
After the last motorcycle cop is killed, Harry returns to his trashed car. Briggs appears and pulls a gun on Harry as he leans into the car to pick up the bomb. A small boom microphone can be seen through the window behind Harry.
At the pool massacre (during the first few shots of people getting hit by bullets) when the male victim who runs away and gets shoot in the back with the window exploding in the BG, a special effects wire can be seen hooked to him and trailing behind him as he falls to the bottom of the screen.
As Carmine Ricca leaves City Hall at 3:30, and again after the the pool massacre, a 2nd unit cameraman using a Eclair camera is part of the crowd pretending to be part of a new crew. However, he's accompanied by a focus-puller in a white hat (who in fact falls down at about 28:45) which news crews never use.
When Davis kills the Mob Boss in his apartment and kills McCoy in the garage, he can be seen putting a silencer on his .357 revolver. Once the silencer is attached, the shots he fires are very quiet. It is impossible to silence that type of revolver as most of the sound would still escape from the gap that is always present between the barrel and the cylinder.
When shooting on the combat pistol range, none of the shooters or bystanders are wearing hearing protection and only two shooters are wearing eye protection. This is a violation of the most basic and standard safety rules on a police shooting range. This is an obvious over sight because Harry and the "rookies" are wearing the proper protection in the earlier scene when they first meet at the indoor range.
(40. min) The taxi driver drives a hooker and looks at her in the rear view mirror while she hides money in her panties - you can not see things lower than your rear windshield in the rear view mirror (especially not as low as seats are), and even if you adjust it (rear view mirror), you can not see because front seats are in the way.
The Lockheed Electra II mentioned in the airport scene could not have flown "nonstop to Washington DC" from San Francisco as stated. The Electra L-188 had a range of slightly under 2,000 miles and that's roughly 450 miles too short to reach DC.
During the chase scene with the officers on motorcycles, the motor cycles had no windshields. For police officers, it is mandatory for their motorcycles to have windshields. They probably omitted the windshields for stunt drivers.
SFPD Motor Division officers ride Moto Guzzi Eldorado V7 850 Police Specials throughout the film. The SFPDused to exclusively use Harley-Davidson Road King motorcycles and today more units are switching over to the Suzuki DR-Z400S dual-sport bike. They used the smaller and lighter Motoguzzi's for practical filming onboard the ship scenes.
When Davis is assassinating Guzman and his associates, he fires 6 shots during the hit and never reloads. He then shoots McCoy twice in the parking garage. In reality McCoy would have been able to draw his gun and shoot Davis or capture him since Davis' gun would have been empty.
When the cop has the limo pulled over he asks the driver who owns the car, to which the driver says, "He does," indicating the mob boss in the back seat. The car is wearing commercial plates, which would have required the driver to be an employee of the limousine company supplying the car to the mob boss.
After Harry is thrown off the hood of Palancio's car the car windshield runs into the arm of some sort of building equipment, killing Palancio. The arm clearly misses the driver. The dummy in the driver's seat is quite apparent.
When the death squad reveals themselves to Callahan in his garage all he has to do is play along as he has no idea if they plan to kill him or not. Since Callahan is portrayed up to that point as being a shrewd police officer and had suggested to his partner what might happen that he would not have done so makes no sense.
Early in the film when the cop shoots the people at the pool party a girl in the pool gets shot. Presumably hours later as a body bag is being carried away from the scene a red cloudy patch of "blood" can be seen in the pool. It is highly unlikely that the blood would not disperse in the water after all of the time that elapsed and water movement that would have occurred.
During the "try knocking on the door" dialogue, a trolley can be seen passing by on the street outside. One of the trolley patrons waves directly at the camera, obviously aware that a film was being shot.
During Davis' combat pistol round, we see him shoot a stand-up target, then shoulder roll into cover and shoot the target twice more - the last two shots are seen from behind the target. He fires twice more, but we see the first bullet hole pop out before the shot is fired.
In the view of the aircraft cabin, just before Harry stops the plane, lines can be seen attached to pillows and other items in the overhead compartment. These will be used to pull the items down when Harry slams on the brakes.
When Harry hits Grimes head-on with his car, it's an obvious dummy that flips over the motorcycle. Additionally, the legs of the dummy are draped over the motorcycle, but when the camera angle changes, and the actor is laying on the ground, his legs are beside the motorcycle.
It's clearly a cool night when Callahan arrives for the stakeout as everyone inside the store and outside has on at least a light jacket. The only exception is a child reading comic books who is only wearing a short sleeve shirt. It's unlikely that a child would be out at night without at least a windbreaker, especially a cool night.
The goof item below may give away important plot points.
Callaghan visits the forensics's lab after the pimp is shot in the pink car. The bullet tracks (strings) are all aimed in to an individual spot on the passenger seat and come out at a variety of angles. This would require the shooter to jump all over the place, including up in the air.
He was actually shot from a single angle outside the car whilst in the driver seat.