Other mistake: When Ricca is pushing his way through the crowd, just after a man yells "Ricca, you're a killer" a shot of the crowd shows a sign saying "UP YOUR RICCA" which makes no sense. Surely this sign was meant to say "UP YOURS RICCA." (00:03:50)
Character mistake: When Harry walks up to look in the car which Ricca and his men were shot in, he places his hands on the driver's side and back seat passenger's side window sill, while the crime scene is being investigated. No senior Inspector like Harry would do this. (00:09:40)
Visible crew/equipment: When Sweet begins talking to the pimp after pulling him over, a boom mic is reflected in the front windscreen. (00:43:50)
Continuity mistake: When Sweet shoots the pimp, he fires five shots. Later when we see the car being examined, we see six strings representing the bullets paths - only five were fired. (00:44:20 - 00:51:30)
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)
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)
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)
Visible crew/equipment: Near the end when Harry is running from the motorcyle cops, he runs up a ramp and through a door. Below the ramp and just above the wooden wall, a camera and crewman wearing a white hat are visible. (01:46:25)
Other mistake: In the end credits, Callahan is misspelled as "Calahan". (01:56:50)
Visible crew/equipment: When the car crashes into the office building after Harry fires a bullet through the car windscreen, you can see the film crew sitting in the background.
Continuity mistake: When Palancio fires a shot at Sweet during the shootout scene at the warehouse, he is pointing the gun upwards, but when the camera is behind Palancio in the next shot, he is now pointing the gun downwards.
Visible crew/equipment: After Callahan removes the bomb from his 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.
Visible crew/equipment: After the pimp is stopped by the policeman on the motorcycle and shot, the camera pulls back showing the pimp's arm lolling through the car window and the reflection of the camera crew moving about on the back, driver's side wing.
Continuity mistake: When Callahan is in the car with Lieutenant Briggs after they pick up the bomb they drive by the 76 service station three times in about 30 seconds.
Continuity mistake: In the car with Callahan, after picking up the bomb, Briggs puts his glasses on twice.
Continuity mistake: When Harry is being chased down the steep winding street he hits a car with the driver's side of his car. In one subsequent shot, the damage is shown to the passenger side instead.
Audio problem: When Callahan gets out of his car to stake out the robbery, he shuts the car door, and the sound of the door closing is heard after the door is shut.
Revealing mistake: A metal ramp is visible on the front bumper of the 1972 Ford Galaxie sedan when it rams the motorcycle head on.
Visible crew/equipment: When the corrupt copper pulls over the pimp, there is one shot where the camera is zoomed into his glasses. You can see the camera and 2 crewmembers reflected in them.
Continuity mistake: At the end of the movie when David Soul has gone flying into the bay from the aircraft carrier, Eastwood gets up painfully from the deck and pulls his jacket down off his shoulders, and we see that it's bunched beneath his right elbow and upper arm. However, the next shot looking up at him looking down at the water shows the jacket is perfectly straight.
Answer: Because Davis and Sweet demonstrated keen interest, skill and knowledge of firearms (particularly of Harry's .44 Magnum) when they first met. They were flattering Harry, probably in anticipation of inviting him to join their death squad. Harry was impressed and asked for them specifically, even though they were rookies, but he didn't suspect them of being the vigilante killers at that point.
Charles Austin Miller
Yes, he did suspect them. In fact, he suspected Davis from the moment he found out that Davis was the one who reported Charlie McCoy's death. He confirmed his suspicion about Davis after he compared similarities between bullet that killed one of the victims and the one he purposely misfired from Davis' revolver during shooting competition and retrieved it afterwards. He explained it all to Early Smith right after raid on Palancio (and thus unwittingly sealed his partner's fate).