Other mistake: When they raise the temperature of the room with the facehuggers in, once the room is at body temperature, they open the door onto the much colder corridor. This would have immediately dropped the temp in the facehuggers' room.
Other mistake: Irs stabbed Richard completely through his face and shot him in the torso. Although he fell in the lake, he is later shown on the shore and he is not bleeding. Head wounds normally bleed like crazy. Since he is wounded in two places, he should have drowned or bled out from swimming to shore. (01:19:51 - 01:25:08)
Other mistake: Justin is checking out an online article from one year prior. The article incorrectly says that the arguing couple parted ways "shortly after midnight," when we know that she was in fact killed at 11:46. It may seem that an online article getting something wrong is not a big deal (then again, most movie mistakes aren't), but what makes this funny is that Justin checks a few seconds later the same article, and this time the same line says "shortly before midnight." Someone realised the mistake when it was time to shoot a closer angle of the screen, and production fixed it.
Other mistake: It is three years since the Reapers emerged, but when they reach the Ranger Station, the pick-up truck is still spotlessly clean, and the tyres are still inflated.
Other mistake: As Don is driving towards Washington with Phyllis restrained in the passenger seat, Phyllis keeps on talking about what's going to happen if Don doesn't kill her. Don starts having a panic attack and closes his eyes, even though he is driving. His eyes are shut for nearly 20 whole seconds. How did he stay on the road? (00:28:23)