When entering the town of Fitchburg, Wisconsin, the sign with the population number says "Fitchberg" while the sign welcoming people to the town says "Fitchburg."
In the hospital room with the old woman she says to fix the crucifix. To be a crucifix it has to have a depiction of the crucified Christ on it. Without Jesus it is just a cross.
Dean refers to the magazine in his handgun as a clip.
There is no Fort Douglas, WI, despite the other towns, such as Black River Falls and Fitchburg, being real places. There is, however, a Camp Douglas and Fort McCoy located directly near each other and often mistaken for each other.
In the first scene, when the father turns out the light for the girl, there is a small gap between the overhead light turning off and the "night-time" lighting going on.
At the end when Dean and Sam pull away in the Chevrolet, you can see the whole camera crane which is filming, reflected in the side of the vehicle.