Other mistake: When the main characters are in the motel and they hear the moth flying outside the window, Stu, the creepy deputy with the glasses, picks up a shot gun and racks a round into the chamber. In the next shot of all of people firing at the moth through the window, Stu is now holding his revolver and fires with it. When he runs of out ammo with the revolver he picks up the shotgun again and before firing racks the pump again. Since he already did that the gun should have ejected the unfired shell that was already in the chamber from earlier, but no shell is ejected.
Continuity mistake: In the scene in the army truck Ben Affleck gets a bandage put on his neck. A little while later, when he's outside walking back to that truck and watching the dog, he's wearing a different type of bandage - it goes from a cloth-type to a plastic-type, look for the coloration and size of the gauze part.