When the older gentleman in the Diner greets John, he says "say hello to your old man for me". It is revealed in season 8 that John's father, Henry, "left" and hasn't been seen since John was a child, leaving John quite bitter.
Young John's eyes are a different color than that of the actor who portrayed him in present day episodes, and since people's eyes can't change colors so dramatically, this was most likely an oversight during casting.
In the diner, the Allmann Brother's "Ramblin' Man" is heard playing. The song was released in Sept of 1973 but the date on the paper says April 30, 1973.
The Spanish subtitles translate Samuel's comment "it's about three miles" to "it's about three kilometers." Three miles is about five kilometers.
When Azazel tells Mary, "I'll arrange to have lover boy here brought back breathing," the actor who plays John is clearly breathing.
Although Samuel was disguised as a priest, he comically failed to remove his ample facial stubble.
Dean is seen driving a Ford Pinto with rectangle headlights in 1973. The Pinto had round headlights through the 1978 model year. They didn't have rectangle headlights until 1979-80 model years.
The newspaper shows a date of April 1973 but the song in the background "Ramblin' Man" wasn't released until September of 1973.
Young John says "hook up" when in the diner with Young Mary. Nobody said "hook up" in 1973.