When Scott is dancing with the dog, in the shots from inside the cabin, the collar on his shirt is up. But from the shots outside the cabin looking in, his collar is down.
An actor/stuntman named Steve Dash actually played Jason in almost every scene, but was crestfallen when he found out he wouldn't be given on-screen credit for the role. Due to some studio red-tape, the actor credited for portraying Jason was Warrington Gillette, a stand-in who only appeared on-screen for a brief instance. (Gillette portrayed the "Unmasked Jason" under heavy makeup, who leaps through the window at the end of the film and is only on-screen for about 20 seconds.) Dash was understandably upset, as he put in almost all the work to create and play the character, but got no reward and was only credited as being Jason's stunt-double. When archival footage from this film was re-used in the next two installments in flashback, Dash was finally given the proper credit he deserved and was given sole credit for portraying Jason via archival footage.