Revealing mistakes: In the initial scene when the driver is shot in the head, in order for his blood to have splattered on the window in the manner that it did, the bullet would have had to pass through his head and then through the window.
Errors made by characters (possibly deliberate errors by the filmmakers): In the scene at the police station where Wayne is typing up a report while he talks to Frank, he uses white-out to correct a mistake. But since he's using a multi-part form, the typo would only be fixed on the top copy.