Last night, one of the HBO stations was running a Back to the Future marathon...
OK, so in BttF 2, Marty buys Gray's Sports Almanac in 2015, with the idea of making some money when he goes back to 1985. Doc loses it, and forbids Marty from taking it back. Old man Biff then steals the DeLorean, goes back to his younger self in 1955 and gives him the almanac. This creates a new reality so, when Doc and Marty go back to 1985, it's a cesspool. This is explained in the move as the result of Biff getting rich off the almanac and pretty much buying the town. Fine...
But, a question came to me last night in my sleep-deprived haziness: How was old Biff able to get back to the same 2015 Doc and Marty were in, when he had already changed the timeline by giving young Biff the almanac? He should have gone back to the 2015 off the rich Biff timeline, right? Clearly, it has to be so the DeLorean can get back to Doc and Marty in 2015, so they can go "back home" and see how screwed up it is. I believe that's called lazy writing...
I can't believe I've watched this movie so many times, and have only now thought of this...