Revealing mistake: After the roof blows off the cabin, the hail starts falling inside the cabin. It is fairly obvious that the hail is just being poured onto the cabin. Its most noticeable just after the hail starts falling on the cabin, there is a shot taken from inside the cabin you can tell by the night sky and some of the trees outside that there is no hail in the distance.
Revealing mistake: When Mike is on his way to go vote and he falls down the hill, he gets to a spot where it isn't steep enough for him to be moving fast. If you look closely, you can tell that the editors of the movie fast-forwarded it to make it look like he is rolling down the hill really fast.