Plot hole: Frank Ochoa told miss Nicole that Paul killed 9 people in New York when in fact he killed 10. (00:47:00)
Revealing mistake: When Kersey tries to shoot Wilson at the hospital, he lifts up the table to knock his gun away from him. Kersey fires a shot at the table when it moves up, but a hole doesn't appear in the table. This indicates the gun was loaded with blanks (obviously) and no other effects were done for the scene.
Continuity mistake: In the scene when Paul Kersey meets Geri at the rooftop Japanese restaurant, he drinks from his glass of Dom Perignon and puts his glass down on the table. When the scene changes he is again putting his glass down.
Continuity mistake: Paul Kersey boards a bus where the line number reads 990. When he gets off, the number is 99. He could not have changed buses because the number of the bus itself remains 8027 throughout. (00:54:50)