When Dean shows Sam the article with a picture of a black dog after making a comment about them humping legs, the text on the article suddenly appears backwards after the camera cuts back to it from a close up of Sam.
Robert Johnson, the guitar player at the opening of the episode, can be seen smoking a filtered cigarette. Filtered cigarettes were not a standard until after 1954. As the opening scene takes place in rural Mississippi in 1938, it is highly unlikely filtered cigarettes would be available.
The picture of the black dog that Dean comments on is actually that of Fenrir, a monstrous wolf in Norse mythology, not a hell hound. His destructive powers were so feared that the gods had no choice but to imprison him. Also in the picture is the god Tyr, a deity noted for his courage and strength.