Back to the Future is one of my all-time favourites.
It came up in a conversation with my dad last year, talking about all the little continuity nods in the movie, and throughout the trilogy. E.g. in '85 the Toyota dealership where Marty's drooling over the pickup truck is Statler Toyota; in '55 they're a Studebaker dealer. (In 2015 they're Pontiac dealers; oops!
)
I was surprised that 30+ years on he NEVER noticed what I thought was an obvious one: the name of the shopping mall where Doc and Marty test the time machine. In the original timeline it's Twin Pines Mall:
When Marty goes back to 1955 and flees the Peabody farm he speeds away and takes out one of the pine trees at Mr. Peabody's mail box. Peabody exclaims: "
Why you—! You space bastard, you killed my pine!"
After Marty returns to 1985, the name on the sign at the mall reads...
Lone Pine Mall. My dad never noticed!