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)
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)
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.