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.
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.
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.
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.
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 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.
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).
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.
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.
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.
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.
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.)
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.
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.
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.
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 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.
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.
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.
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.