Foreshadowing / Back to the Future

• Early in the movie, Jennifer and Marty see a black 4x4 Toyota pickup truck, and Marty says how great it would be to drive it up to the lake. At the end of the movie, after Marty has altered history, he discovers that his family now owns the very same truck and they are fine with Marty and Jennifer driving it up to the lake.
• 1955 Doc commenting on how Marty's 1985 photo is obviously a forgery, since the top of his brother's head has been cut off.
• One of the clocks seen at the start of the film shows a man hanging off of the minute hand (on the surface, a tribute to Harold Lloyd (no relation) in the climax of Safety Last!). Doc is later hanging off the clock tower face towards the end of the film. Also, the clocks are slow, with Marty being told of this, which sets up that he is going to go back in time later.
• Marty's skateboard at the beginning hits a case of plutonium when Marty enters the garage. As revealed later, Doc had stolen it from the Libyans who later shoot him dead. Before that, Doc's TV was automatically turned on, which shows the news talking about a stolen case of plutonium.
• The following exchange at the beginning:
  Strickland: Now let me give you a nickel's worth of free advice, young man. This so-called Dr. Brown is dangerous. He's a real nutcase. You hang around with him, you're gonna end up in big trouble. [...] No McFly has ever amounted to anything in the history of Hill Valley!
  Marty: Yeah, well, history is gonna change.
• Biff in 1985 telling Marty to say hi to Lorraine for him may seem like a clumsy attempt to be nice, but it's meant to foreshadow that in her better days, Lorraine was very good-looking and that Biff has a Villainous Crush on her.
• Upon being rejected for the Battle of the Bands, Marty says "I'll never get to play in front of anybody..." Guess what Marty does at the dance in 1955?
• While recollecting how they first met, Lorraine has no idea what George was doing when he got hit by her dad's car. She presumes it was bird-watching but he doesn't confirm this. Later, Marty learns first-hand that what he was really doing was something other than bird-watching.
• The McFly family is watching the episode of The Honeymooners where "Ralph dresses up as the man from space". The Baines family in 1955 watch the same episode when Marty is with them. It also sets up Marty being mistaken for a "man from space", first of all in front of the Peabodys when Marty first arrives in 1955 and later when he turns up in George's bedroom to coerce him to take Lorraine to the dance.
• When Marty gets to 1955 and crosses the street in front of the movie theater in the town square he is nearly hit by a car. Guess what happens a few scenes later?
• After the chase around the town square, Lorraine tells Marty that a man should stand up for himself to protect the woman he loves. George later does exactly this on the night of the dance, standing up to Biff for the first time in his life to save Lorraine.
• Marty trips up Biff in the diner, and then punches Biff in the face before the chase around the town square. Near the climax, George stands up to Biff by punching him in the face, knocking him out.
• When Marty tells Doc near the end that he never saw his dad stand up to Biff in his life, Doc's concern is piqued. Astute viewers can cue in that the 1985 that Marty will return to may not be quite the same in noticeable ways...
• When Doc tears up the letter Marty wrote to his future self, he shoves the pieces in his pocket instead of throwing them away.
• When Marty and Doc test the plan to send Marty back, the test car drives through the movie theater at the end of the block after being electrocuted. Guess what happens when Marty comes back to 1985.
• The first time we see Doc get shot by the Libyans, he gets shot dozens of times and actually gets knocked backward as he falls. The second time we see this, he falls much faster in a staged-looking manner. It turns out in the revised timeline that Doc was wearing a bulletproof vest and was playing dead, thus his second fall was fake.
• "If Grandpa hadn't hit him, then none of you would have been born." And that's precisely the scenario that Marty creates later on.
• A retroactive one for Part II. Just before the Libyans attack, Doc remarks that his trip to 2010 means, among other things, that he'll be able to learn who wins the next 25 World Series. In the next film, Marty and Doc will learn the hard way what happens when someone abuses future sports knowledge.