Continuity mistake: In 'A Very Supernatural Christmas' (S3E08), when the Gods are beginning to sacrifice Sam and Dean, they slice Sam's right forearm. However, at the end of the episode, Sam has his left forearm bandaged, not his right.
Revealing mistake: When the boys are tied up and the pagan gods are preparing for the ritual, the second time they cut into Dean's arm the knife is pressed along his arm and the blood is removed showing no cut at all.
At the beginning, the cabin looks very similar to the cabin in The Evil Dead, even the swinging bench (although on the opposite side of the door). And the girl calls out the name Ashleu, Bruce Campbell's character in Evil Dead.