Plot Hole I don't think I've seen mentioned before....
Why should the 1985 Doc (who is stuck in 1885) need to look at a picture of his tombstone with his death date and the "matter of $80"? He already should KNOW this because his former self, 1955 Doc, already knew this after he and Marty discovered the tombstone in 1955, so this memory should have automatically transferred itself to 1985 Doc.
And notice 1985 Doc also says "Now I wish I'd paid him off"... well he should have already KNOWN this was going to happen, so why didn't he just pay Buford off to begin with and avoid all this before Marty even arrived? Better yet, why didn't 1955 Doc just let Marty travel directly back to his own time of 1985, and say "It's okay Marty, I will just make sure I remember about this Buford Tannen problem, and avoid it at all costs when I get to 1885...".
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That's putting it mildly, OO7