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.
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)
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.
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.
Revealing mistake: During the shootout on the Jet, the second hijacker hides behind a wall and Harry shoots through the wall, killing him. When the bullets supposedly go through the wall, one of them puffs out black powder and marks the wall, showing it's really an explosive charge going off and not a bullet.
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.
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.
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.
Character mistake: The coroner says the pump was shot with a full mag from a magnum which is six rounds but the pimp was only shot with five.
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.
Other mistake: In the final credits, DiGiorgio is misspelled as "DiGorgio."
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)
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).