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: When the girl goes to the bathroom she drops her cigarette into the sink, you see it bounce and go to drop into the drain, but in the next shot of it is lying across the drain.
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.
Revealing mistake: When the two women first enter the sheriff's station there is a dead deputy on the floor with a Beretta 92 in his right hand. There are two mistakes in this scene. First, he supposedly died firing the gun, but the hammer is decocked which has to be done manually. Second, one of the shots shows three spent cartridges on the ground. The shells have brass crimping opened on the front. Real shells have no need for a crimp because of the lead bullet. It is obvious that they fired three blanks out of the gun and used those shells in the shot.
Audio problem: In the motel, the police are unnerved by a giant, deadly moth-creature banging against the window panes, and they open fire with everything they've got at a range of about ten feet. Strangely, we only hear gunfire sound effects when the camera is looking directly at the police officers firing their weapons. When the camera repeatedly cuts to close-ups of the window (to show the ongoing damage), there are no gunfire sound effects, even though we see and hear the window still popping and shattering under the hail of bullets. This same, awkward, delayed-gunfire sound editing occurs elsewhere throughout the film, as well.