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: 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.
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.
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.
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)
Continuity mistake: When Harry hits Grimes head-on with his car, the car comes to a rest and Grimes' legs are draped over the motorcycle, but in the next shot, Grimes is lying on the ground and his legs are beside the motorcycle.
Continuity mistake: During the supermarket scene, one of the thugs points his shotgun at Early Smith and tells him to "Freeze". At this point you can see the till is open, but after a quick cut to Harry, we return to the cash desk and the till is now closed.
Continuity mistake: When one of the hijackers says "Now close that door!" to Harry in the aeroplane, he is pointing his gun directly at Harry, but in the next shot he is pointing his gun downwards.
Continuity mistake: In the front view of the pimpmobile crossing the Golden Gate Bridge, we see that the door mirror has a shroud. This is the correct mirror for that car. Then, we see the pimp looking through the mirror at the traffic following him. That mirror has no shroud - it is the wrong mirror for that car and does not match the view we saw from the front of the car.
Continuity mistake: After the pimp is killed, Harry and his partner follow a couple of cars across the Golden Gate Bridge back toward the crime scene. The sky changes between shots of the vehicles.
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.
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.
Continuity mistake: When Harry, dressed as an overseas pilot, enters the aeroplane during the hijack scene, one of the hijackers draws his gun right in front of Harry when he says "Good afternoon". In the next scene, the gun is further away from Harry than it was in the previous shot.
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).