When Dale throws the knife at the skunk, we see it hit the tree or perhaps the skunk low towards where the head is, but when he goes to take the knife back he takes it from way above and to the right of the skunk.
When the teens are driving in the van, every time we see the girl in the front seat from the side, her window is open and blowing her hair around, but when we see them from the front, there is no wind and they are quite clearly not driving. The driver even does the old "constantly moving the wheel" bit.
In the cave near the end, there's lots and lots of light coming in. Apparently it's meant to come in through an entrance, but it's night outside. And for some inexplicable reason the light is flickering too.
When the kids drawings are revealed in the beginning, the kid clearly knew how to draw the word 'mommy'. The second drawing shows he rotten 'mommy' with only ONE 'm'.