As Coffey and Bates patrol in their unit, which is prominently marked as 2203, they come upon Belker's abandoned car. While they speak with each other, the interior shots of the car reveal that the windshield has no rear-view mirror on it. An exterior shot of the car as its point of view approaches straight on towards the camera without any indication of the unit number, now shows a rear-view mirror on the windshield. Then in the following exterior shot when Coffey and Bates exit the car, point of view camera angle from the side that clearly shows the unit's number of 2203, again shows no rear-view mirror attached to the windshield.
When the police are looking around the older woman's apartment, there's a large blood stain above and to the side of the window. However, there was also blood on the fire escape outside, even though she only fired one shot.
When the bum fires Balker's gun in the basement, a couple of seconds elapse between the muzzle flash and the "recoil", indicating he was firing blanks, and jerked the gun back himself. This is followed up by the bum saying "That's some kick", which makes it even more obvious.