Continuity mistake: When Marty is at the Enchantment Under the Sea dance and is about to disappear, he puts up his right hand and it starts to disappear. On the very next shot of him, he is putting down his left hand. (01:25:35)
Continuity mistake: During the Johnny B. Goode song, the mike cable's position keeps changing depending on the shot, despite no-one touching it. (01:27:30)
Continuity mistake: When Biff's car slams into the manure truck, in the first shot the side of the truck is adjacent to the car's back passenger door, but in the next overhead shot the side of the truck is adjacent to the front windshield. (01:07:20)
Revealing mistake: When Biff and his gang are chasing after Marty in their car around the Courthouse Square, in the shots that takes place in the Courthouse yard, you can see that it's Biff's stunt double that's driving. (01:06:45)
Continuity mistake: Before his parents kiss, Marty's right hand begins to disappear. As he slumps to the floor both hands are visible holding his guitar. (01:25:35)
Revealing mistake: Marty finds Doc's name in the phone book; when he sees the name, then runs his fingernail over to the phone number. Look closely at the phone book; it has several creases across the same place where Michael J. Fox ran his fingernail from previous takes. (00:36:15)
Continuity mistake: When Marty is being chased on the scooter turned skateboard, he is hanging onto the corner of a blue truck. Biff tries to smash him between the two, and the camera shows him pulling himself along the side of the blue truck, then hanging off the back corner again. (01:04:10)
Continuity mistake: When Biff is grousing to George about who will pay his cleaning bill (for spilling beer on his jacket when he wrecked George's car), there is a picture frame on a stand in the background. It's initially facing fairly rightwards, but in the next shot of the argument, it's facing more towards the camera. (00:11:55)
Continuity mistake: In the escape from Biff on the skateboard, Marty collides with a couple near some steps. His skateboard is left at their feet, but after a cut his skateboard's now behind the woman's head in an ideal place for Marty to grab it. The papers thrown in the air suddenly multiply and appear on the ground (under the people, which means they must have dropped the papers first before falling over!). (01:04:15)
Continuity mistake: When Marty and George are in the cafe talking about Goldie Wilson being mayor, George's head is turned facing Marty, but the next second his head is turned the other way. (00:40:35)
Continuity mistake: Towards the very end of the film when George and Lorraine are unpacking George's new book, you will notice in the close-up of Lorraine's hands that they are actually someone else's for that shot. They are very aged and much larger than her real hands, which you can compare right away when the camera goes back to full view. (01:49:25)
Continuity mistake: When Michael J Fox's parent kiss and the family picture (with everyone standing on the grass) gets fixed, Michael places the photo on the guitar, gets very happy and starts playing the guitar enthusiastically. Seconds later, a brief shot of the guitar shows the picture with Marty only, his brothers are missing.
Continuity mistake: In the scenes where Marty enters the Lyon estates, if you look close you can see a stop sign, but if you watch the next two movies the stop sign has been removed. (00:11:50)
Continuity mistake: Near the beginning, right after Einstein time travels, Doc walks up to the time machine and says it's cold. He then opens the door and greets Einstein. In this split second you can see Einstein's stopwatch has a non-illuminated LED display, but in the next shot the face has changed to the bright red numbering. (00:23:10)
Continuity mistake: When Marty arrives at the Twin Pines Mall at the beginning of the film, he drops his skateboard behind the DeLorean. When Doc starts the car with Einstein in it, the skateboard disappears. (00:19:05)
Continuity mistake: When Marty leaves the Peabody farm and turns onto the paved road, it has solid white lines marking the sides of the road. A few seconds later, Marty is shown driving down a road without white lines on the sides, even though he's apparently only driven a short distance down the road. (00:33:30)
Audio problem: When Marty's band is auditioning at the beginning of the movie, the drummer's hands are inconsistent with the drumming sounds. Especially obvious when they get cut short and we hear a mini drum roll, which the drummer definitely doesn't play. (00:08:15)
Continuity mistake: During dinner, when old-Lorraine says "We all make mistakes in life, children", she swaps from having one hand on the glass to both hands, between one frame and the other.
Continuity mistake: When Marty rides to the mall he is riding his skateboard, but after Doc comes out in the car, it's gone, then at the end of the film when Doc drops off Marty he has it again. (00:18:40)
Continuity mistake: When the DeLorean sets off for the lightning strike, you can see it leaving tracks and spray on a wet road, later there is a close up of one of the wheels, which shows the road to be dry. Throughout this scene the road appears to alternate between wet and dry, during what is supposed to be a storm. (01:39:00)
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.