1. When Marty goes back to the future for the first time, he vanishes into The Atomic Kid.
According to Back to the Future co-writer and producer Bob Gale, in the first two drafts of the screenplay, "...the whole clocktower sequence at the end wasn't there: It was going to be the radiation from an explosion at a nuclear testing site that would power the time machine back to 1985." The Atomic Kid is a 1954 movie about a man who is exposed to nuclear radiation on an atomic bomb test site.
2. Doc hanging off of the clock tower is foreshadowed in Back to the Future's opening credits.
3. The clock tower clock is unloaded in the background of Back to the Future Part III.
4.Jaws 19 in Back to the Future Part II is directed by Max Spielberg, the son of the Jaws director and Back to the Future producer.
5.And the slogan for the Jaws 19 is "This Time It's Really Really Personal."
6. The beer George drinks changes with his confidence.
7. The drummer at the school dance in 1955 plays in a swing style because he doesn't know about rock drumming yet.
8. Marty has the same experience in every new version of the Courthouse Square.
9. The property Marty crashes into when he time travels for the first time belongs to the Peabody family, who have a son named Sherman. Mr. Peabody and Sherman were time-travelers in The Adventures of Rocky and Bullwinkle and Friends.
10. Biff's casino requires smoking. LOL.
11. 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
12.The Asian population of 1887 Hill Valley is shown in the background multiple times. Considering that the Western side of the transcontinental railroad was built almost entirely by Chinese immigrants and a mid-construction railroad is a major plot element of Back to the Future Part III, not to mention the important fact that Chinatowns were a major part of the Old West, this makes perfect sense.
13. The beginning of Part II was a shot-for-shot remake of the end of the first movie.
14. In the second movie, Doc has two slots for 1864 money because the Confederacy had its own currency during the Civil War.
15. The judge who tells Marty that "The Power of Love" is too loud is played by Huey Lewis himself.
Maybe this was obvious in 1985, but time has turned it into a subtle detail.
16. 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.
17. At the end of the third movie, Needle's gang is made up of members of Biff's posse from each time period.
18. The clock and toilet Doc mentions in Back to the Future are present in Part III.
19. Hill Valley's theater is under construction in 1885.
20. This newspaper in Biff's 1985 shows a very different world.
21. 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.
22. One of those predictions was there being a cure for cancer.
23. In Back to the Future Part III, Marty learns from his past and checks to see if he's wearing pants before getting up.
24. In Part III, Doc inadvertently describes the first time Marty traveled through time.
25. The influx of malls is shown in Back to the Future.
26. Doc wears multiple watches, because he freaking loves time.
27. 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.
28. The clock tower's damage is consistent.
29. And it's still broken in 2015.
30. And that guy Marty's talking to is 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.
31."In Back to the Future III, Doc says that a suitable device to repair the time machine won't be invented until 1947. That was the year the transistor was invented at Bell Labs."
32.In Part III, Doc gives the time-machine train a white-wall-tire look, so it looks like the DeLorean.
33. To make Marty's son, played by Michael J. Fox, and Biff's son, played by Thomas F. Wilson, look different from their fathers, they have new eye colors.
34. Doc is able to spend so many years doing experiments that make no money because he sold his family's estate.
35. 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
36. In Part III, Doc has fully painted his model, unlike the first movie.
37. The license plate on the 2015 DeLorean is a barcode.
38. 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.
39. The Court House is a mall in 2015 because lawyers are no longer a thing.
40. The photographer in Back to the Future Part III is actually the trilogy's director of photography, Dean Cundey.
41. When Marty's Uncle Joey, a convict, is a baby, he's wearing black and white stripes.
Marty says to him, "Better get used to these bars, kid."
42. The diner in 1955 is a fitness center in 1985.
43. In Part III, you can see Clara waiting for Doc in the background.
Back to the Future 3 (1990) when Doc and Marty are planning where to get the Delorian up to 88 mph, you can see Clara in the background waiting to be met by Emit. Since he decided to not meet her she took a wagon on her own and almost crashed into the ravine forcing Emit to save her. from MovieDetails
44. A tiny and great detail in the first movie: Marty has a guitar pick in his pocket.
45. Due to inflation, a Pepsi in 2015 costs $50.
46.The McFly house in 1985 is the same as the house on the billboard in 1955.
Back to the Future(1985) The billboard that Marty sees in '55 is an exact image of the McFly home.