When Killer is in his car staking out the old man's apartment, you hear the sound of him cocking his semi auto pistol, which takes two hands. As this takes place, you see him put the gun into his holster with one hand.
When Driver kills his first victim in a point blank range, the cubicle wall behind the victim is splattered with blood. Later when Cop is watching the surveillance footage of Driver and the victim, the blood splatter on the wall is smaller.
Towards the end of the film, when Cop and Killer are talking, in a shot from behind Killer his coat collar is turned up. The next shot, in front of Killer and behind Cop, the collar is now turned down and is so when Killer turns to walk away.