Audio problem: Lorraine comes to see Marty in Doc's garage in 1955 to ask him if he will go to the dance with her. Just before Lorraine moves to the other side of the car from where she walked in, she begins to say something to Marty. The problem is that her lips don't move when she talks, I guess the right clip ended up on the editing room floor. (01:11:40)
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)
Audio problem: When Marty is playing the intro to "Johnny B. Goode" you can hear some cymbals in the background. But you can see behind him that the drummer isn't hitting any. (01:27:30)
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.