When Dean is telling Sam about Sonny, he says "You remember when we were kids, that spring in Upstate New York? Dad was on a rugaru hunt." Yet in season 4 episode 4 "Metamorphosis", when the hunter Travis tells them that Jack Montgomery is a rugaru, Dean replies with "Is that made up? That sounds made up."
The holes in the barn door don't match the tractor tines either. There are only four holes when there are seven tines, the holes are very spaced and they are quite centered on the right door, whereas the bloody tines are the ones one the right (when facing the tractor). There is no room on that door for the three remaining tines on the left.
When Dean goes to the barn to inspect the death scene, the holes in the barn door don't match the ones at the beginning: there were only three at the time and they were also very near to the center of the twofold doors (as that's where Jack backed away). In that scene there are four and they are in the center of the right door.
When Jack gets killed at the very beginning, the tines of the tractor should come out of the barn door at the same time because they are all at the same level and the tractor is going straightly forward. However, we actually see the first, then the second, and finally the third one piercing through the wooden door.
When Sam is looking at the wall where the athletic awards are hung, on Dean's wrestling champion award, the word "Athletics" is incorrectly spelled as "Atheletics"