Continuity mistake: When Marty is on a skateboard being chased around the town square by Biff and his friends, he grabs the back end of a truck. He does with two hands at the same point, then uses one hand in the next shot, then two hands again.
Continuity mistake: As Marty watches George pedal away from the café, the amount of sunlight cast on the town square decreases greatly between shots. (00:41:00)
Continuity mistake: As Lorraine's family watches "The Honeymooners" around the dinner table, a round pillow on the sofa nearest the lamp repositions itself. (00:46:05 - 00:47:30)
Character mistake: In the parking lot when Marty begins to film, Doc says the date and exact time according to his watch. Marty then looks at his watch. Apparently his is off because he shakes his wrist and puts the watch to his ear to see if it is still working. However, he is wearing a digital watch which wouldn't tick or likely have broken parts to rattle.
Deliberate mistake: The sequence with the Lybians chasing Marty at the end of the film is much shorter than the original chase.
Continuity mistake: In 1955, when Marty shows Doc the flux capacitor, he points the flashlight in different directions between shots (first at Doc, then the flux capacitor). (00:51:35)
Continuity mistake: After Old Man Peabody fires his rifle at Marty, it changes position between shots. Initially the rifle is trained on the barn doors, but when the DeLorean busts out a moment later he's got it aimed at the ground instead. (00:33:05)
Continuity mistake: As Marty takes in his surroundings at the 1955 town square, he pauses for a few seconds to glance at the Roy's Records storefront. In that short span of time, the cloud cover changes entirely. (00:35:55)
Continuity mistake: When Marty pulls the family picture from his shirt pocket in front of George's house, he holds it with both hands at first. But once the angle changes, only his right hand has a grip on the picture. (01:01:45)
Continuity mistake: When George walks to Lorraine with the flirting words written on a notepad, her purse changes positions between one shot and the other.
Revealing mistake: While Biff and the others are chasing Marty on the skateboard, when Marty grabs a green truck the stunt man is doing it for him.
Continuity mistake: When Marty takes a swig from the bottle, we see the motion of Lorraine's hands as she lights a cigarette. She finishes lighting and then grabs it with her left hand. In the next shot where Marty spits alcohol all over the place, Lorraine is busy lighting her cigarette a second time. (01:18:10)
Continuity mistake: As Doc reacts to the icy DeLorean, the door Doc was about to pull open barely has any ice on it, but when Doc opens the door with his foot, more ice is on it, then melts after Einstein runs back into Doc's truck. (00:24:00)
Revealing mistake: As the DeLorean's vanity plate spins around in place, you can see how it's attached to a short steel post, stuck into a hole in the pavement. (00:22:10)
Continuity mistake: When the clock tower bell sounds a third time, Doc backs away from the ledge and stares down below in shock. Doc's arms are hanging down at his sides, but once the shot changes he's seen cupping his ears. (01:35:35)
Continuity mistake: As Doc turns the plutonium jar on the back of the DeLorean, the little thing sticking out of the jar slides greatly down, without any motion of moving. (00:27:15)
Audio problem: Marty's singing during the "Johnny B. Goode" number is obviously dubbed; it goes out of sync several times. (01:27:40)
Chosen answer: I think that is *exactly* what he is thinking. He realizes that by standing up to Biff, George may have irrevocably changed his personal future, and therefore affected Marty's future as well. This is exactly the sort of thing Doc was so eager to prevent by refusing to hear any information about the future.
Phil C.