Continuity mistake: Motorcycles used by traffic officers throughout the movie are Moto Guzzi V7 850. In the scene on the docks, two officers arrive on Moto Guzzis, but the moment they ride up the narrow ramp, they suddenly ride Triumph Bonnevilles. The two Triumphs are slightly different. Trained eyes can tell that the traffic cop and Callahan use them interchangeably, switching back and forth a few times. (01:45:45 - 01:52:55)
Other mistake: The vigilante bike-cop slaps a supressor on his Colt Python. A suppressor on a revolver doesn't work without serious modification. There's only one revolver I know of which would be able to use a suppressor, namely the Russian Nagant M1895. And that's due to the gas-seal system in the cylinder, which seals all the out-gassing, only letting the gases out the muzzle-opening. (01:01:00 - 01:10:00)Budoshi
Revealing mistake: When the girl falls out the hotel window after one of the officers shoots her, you can see that the girl is an obvious dummy.
Audio problem: When Harry is talking with the crime scene technician, they are looking at the bullet trajectory in the pimp's car and a distinct female sneeze and cough can be heard in the background.
Factual error: The woman walking past waiting fares, saying "excuse me, this is important." She gets into the right rear door of a 1963 Chevrolet, with flat door sill. In the interior of the car there is "Kick Up" in the doorsill, and a Ford door release handle, flush with the side trim. The 63 Chevy had different handles.
Revealing mistake: When Harry is thrown off the hood of Palancio's car, the windshield runs into the arm of a crane, killing Palancio. At this point, the arm misses the driver, and the actor playing Palancio has been replaced by a dummy.
Visible crew/equipment: In the scene just after the pimp is shot by the cop in his Cadillac, the right side of the car is shown as the cop walks back to his motorcycle. The filming crew is visible moving around in the reflection of the car paint of the right side quarter panel, in spite of the paint being dulled to reduce the reflections. (00:46:25)
Continuity mistake: The cop walks up the hill to the pool party scene with 2 pipes and the bomb satchel in his hands. When he gets there the pipes magically turn into a machine gun.
Revealing mistake: Before the pimp is shot dead by the police officer, a circular discoloration is visible on the actor's neck, exactly where he is about to be shot.
Continuity mistake: When Early Smith is confronted by a thug with a shotgun in the store, the number on the till is "3.21", but a few shots later changes to "1.79".
Continuity mistake: When the thug with a shotgun tells Early Smith to get down on his knees during the supermarket scene, you can see a female assistant standing next to Early, but when Harry shoots the thug dead, the assistant has disappeared.
Continuity mistake: When Early Smith runs out of the supermarket and shoots one of the three thugs, he lowers his gun after killing the thug. In the next scene, he is holding the gun back in a raised position.