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: 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)
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.
Continuity mistake: When the pimp is being followed on the Golden Gate Bridge by one of the motorcycle cops, the shot starts near the center span but in the next couple of shots, they are at the beginning of the bridge.
Continuity mistake: 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.
Continuity mistake: When the cop pulls over the mob's limo, he parks his motorcycle perpendicular to the vehicle, but in the next shot it is parked facing in the same direction as the limo.
Revealing mistake: At the very end of the film Clint's corrupt boss drives away from the docks. He does this at a certain speed. Just before the time bomb goes off the car goes at a much slower pace and also Clint's boss has changed into a dummy.
Visible crew/equipment: The camera and cameraman are reflected in the glass in the shooting practice room.
Continuity mistake: As Clint Eastwood hunts down the last store robber, he moves swiftly from behind the end of a row of shelves, quickly aims and shoots number three hood dead. This is filmed from behind and his stance is legs spread akimbo. But in the next shot, looking back at him with gun raised, his right leg is braced forward and left backward.
Revealing mistake: When Clint crashes head-on with the bent Copper on his bike (near the end of the film) It is obvious that a dummy stand-in Cop is used for the 'moment supreme'.
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)
Other mistake: After the pool massacre, when the shooting stops, at the left of the screen and right off the policeman's helmet, a man sits down and then lifts the beach chair up.
Other mistake: When Callahan meets Sunny in his apartment lobby and she asks him, "What does a girl have to do to go to bed with you?" Callahan replies, "Try knocking on the door." and if you look behind Callahan at the Trolley going down hill, you can see a kid waving to the camera.
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).