Revealing mistake: Throughout the movie, we are reminded repeatedly that Harrison and Travis are naive 9-year-old boys who have absolutely no knowledge of guns or gun safety. Near the end, when the two boys are trapped in the backseat of the cop car, they clumsily try to blow out the window with a 40 caliber semi-auto handgun, but they can't even figure out the safety mechanism. When they do release the safety moments later, young Travis suddenly takes aim and fires with a perfect two-handed forward presentation. We see the gun and Travis's hands in close-up, and he's properly holding the semi-auto with trained dexterity just before it fires. This appears to be a skilled stand-in firing the gun.
Cop Car (2015)
1 revealing mistake - chronological order
Directed by: Jon Watts
Starring: Kevin Bacon, Camryn Manheim, Shea Whigham, Hays Wellford, James Freedson-Jackson
Genres: Thriller
Factual error: The Sheriff clears all other police traffic on a specific radio frequency so that he can "secretly" chat with the two young boys. Of course, secrecy is crucial to the Sheriff's motivation all throughout the movie. However, police radios have extraordinary range, and police radio channels are constantly monitored by neighboring police forces, by other first-responder agencies, by civilian police scanners, and even by the news media. The Sheriff should surely know that it is impossible to conduct a "secret" or even "private" conversation over police radio.
Sheriff Kretzer: You don't... Steal... A Fucking... COP CAR.