Cop Car (2015)

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.