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'.
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.
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.
Other mistake: When Harry and Early Smith are in Lt. Briggs' office, Briggs' first name on the desk nameplate is spelled "Neil", but when Harry and Early Smith leave the office, Briggs' first name on his office door is spelled "Neill."
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.
Visible crew/equipment: When Harry returns to the trashed car after the last motorcycle cop was killed, he leans into the back seat to look for the bomb and Briggs pulls a gun on him. At this point, a small boom microphone is reflected in the window behind Harry.
Visible crew/equipment: When Harry is running up the steps to enter the aircraft carrier, you can see a camera and crewman in a white hat below, just above the wood wall.
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.
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.
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.
Other mistake: Before the shooting, we saw a reflection of the officer in the outside mirror of the limo. The glass is rectangular. The actual mirror of the '73 Cadillac Model 75 had rounded corners.
Revealing mistake: Near the beginning, a couple of men are shot in the back of a car. The guy in the middle, Ricca, is supposedly dead but takes several normal breaths as the shot lingers.
Revealing mistake: Multiple times during the final chase, tire marks on roadways are visible where multiple takes of the stunts happened.
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).