In this episode, Henry leaves John while he is still a young boy. His departure from John's life and John's ensuing resentment are major plot points in this episode. However, in "In the Beginning" (season 4, episode 3), in Jay Bird's Diner, a man walks in and greets John with, "Hey Winchester!" At the end of their conversation, the man says, "Say hello to your old man for me." John replies with, "You got it, Mr. D," This presumes that John's father is still alive in the 70's when he has actually already served in the war at this point, so he's definitely into adulthood with his father still around.
John also describes himself as "a mechanic from a family of mechanics". We never hear about John's mother, and it is possible that she remarried, therefore giving the possibility of a stepfather who is a mechanic. Or it is also possible that this is just a lack of continuity on the part of the writers.
When the owner of the new age shop pulls the gun on Henry, her movements and the sound it makes when she cocks it insinuate she has a pump shotgun however, the weapon she's holding is a single shot breakdown style not a pump and would not have made that sound nor required that motion.
Henry refers to Sam and Dean's tactics as "alpha male" behaviour but that phrase wasn't known in his time (it was coined in 1944 but not popularized until post-1968).
Henry looks at a license plate sticker to determine the year but those would have not been around in his day.