1. The clock tower clock is unloaded in the background of Back to the Future Part III.
2. The beer George drinks changes with his confidence. I know! What a detail!
3. Doc's shirt in Back to the Future Part II seriously foreshadows Part III.
4. And the shirt finds a new use in the third movie.
5. Marty has the same experience in every new version of the Courthouse Square. I love that kind of thing so much.
6. Biff's casino requires smoking. LOL.
7. The drummer at the school dance in 1955 plays in a swing style because he doesn't know about rock drumming yet.
8. The same family runs the transportation business in Hill Valley for 100 years. Well, for at least 100 years.
In Back to the Future the Statler family ran the transportation business in Hill Valley. In 1885, Joe Statler's business was called Honest Joe Statler, which advertised "fine horses sold, bought, and traded". In 1955, it changed to Statler Studebaker. In 1985, it was Statler Toyota. from MovieDetails
9. This newspaper in Biff's 1985 shows a very different world.
10. And Part II's 2015 newspaper made some predictions.
Back to the Future Part II (1989) makes several predictions in its fictional 2015 newspaper, such as a "Queen Diana", "Tokyo stocks up" (due to Japan's then strong economic growth), a female president, and "thumb bandits" hinting the fingerprint payment system shown later in the film. from MovieDetails
32 years after this movie came out and still no female president. How grand.
11. One of those predictions was there being a cure for cancer.
12. In the third movie, Marty references Clint Eastwood in 1955, too early for Doc to know who that is, but a poster for Clint Eastwood's first movie is next to Marty when he says it.
13. The clock tower's damage is consistent.
14. And it's still broken in 2015.
15. Oh, and that guy Marty's talking to? He's the mechanic in 1955!!!
The mechanic is played by Charles Fleischer, who voices Roger Rabbit. Who Framed Roger Rabbit is another movie directed by Robert Zemeckis.
16. The diner in 1955 is a fitness center in 1985.
17. In Back to the Future Part III, Marty learns from his past and checks to see if he's wearing pants before getting up.
18. Doc is able to spend so many years doing experiments that make no money because he sold his family's estate. What a perfect explanation.
19. The Court House is a mall in 2015 because lawyers are no longer a thing.
20. In 1955 in Back to the Future Part II, there's an advertisement for a trip to Cuba – 7 years before the Cuban Missile Crisis.
21. When Marty's Uncle Joey, a convict, is a baby, he's wearing black and white stripes. Everything is something in these movies!
Marty says to him, "Better get used to these bars, kid."
22. Hill Valley's theater is under construction in 1885.
23. The license plate on the 2015 DeLorean is a barcode.
24. Doc hanging off of the clock tower is foreshadowed in Back to the Future's opening credits.
25. When Marty gets back to 1985 in the third movie, the ravine is now called the Eastwood Ravine because people think "Clint Eastwood" (Marty) fell to his death.
How none of these movies won Best Original Screenplay I will never understand.
26. In Part II, the 2015 antique store is filled with specific gems.
In Back to The Future II (1989), the antique store is full of easter eggs; a VHS copy of Jaws (directed by BTTF's executive producer Steven Spielberg), a JVC camcorder (used by Marty to record Doc's experiment in the first film), and a Who Framed Roger Rabbit doll (directed by Robert Zemeckis). from MovieDetails