Continuity mistake: McCaleb sees the suspect parked across the street in a car, he then takes the shotgun from the trunk of his car. Detective Winston then exits the car and is seen at the rear of the car saying "You're killing me here, what's going on?". McCaleb then shoots up the suspects car. At the end of the scene Detective Winston is then seen getting out of the car for a second time, despite the fact that she has already got out of it once already.
Plot hole: Clint Eastwood's whole investigation/solution is based on him suspecting that the victims had a common tie, rather than being random. The problem is there is no reason for him to think this - there is no reason given for them to not be considered random victims. For him to think otherwise was a bit far fetched. In the book, however, they were all on the organ donor list, which led him to the fact they all had the same rare blood type and so on.