Does Biff alter the timeline twice?
When Biff scolds his teenage self for being careless with the almanac, he mentions using a safe but corrects himself, realizing that the boy does not own such a thing. This suggests that the corrupt millionaire has already had a turn but lost his status. In that case, though, the villain is trying to regain what he should have no memory of. I'm not sure if this is a writing error or if it means that the audience does not see the first time that Marty, Doc, and Biff go to war over time travel. If it's the latter, then maybe the horrible man remembers what he has done because its source is in the future, which is purely theoretical. Supposing that Biff's line is not a mistake, it might exist to show the gap between reality and the capability of the DeLorean. It could even say that the nightmarish 1985 is parallel to the original one, and Marty and Doc are stuck in it simply because they have enabled the simulation of 2015. The same would apply to Einstein and Jennifer.share