Ok, this has been bugging me for years and years and years, and I've never seen anyone mention it in all the time the movie's been around.

Doc takes Marty and Jennifer out of 1985 to meet their future kids and stop them from going to jail. Sounds fine. BUT, if 1985 Marty and Jennifer leave 1985 to travel 30 years in the future, how in the world can they encounter their future selves??? People in 2015 would say Marty and Jennifer disappeared 30 years ago and haven't been seen since! Did they somehow go to an alternate future where Doc never told Marty and Jen to come with him? If so, how did that happen? Biff made the 'flying DeLorean?' remark, and watched it disappear, right? At that point, 1985 Marty and Jennifer are in the future. In 2015, Biff again makes the flying DeLorean comment and says he hasn't seen one of those in 30 years, so it had to be on the same timeline.

I love the trilogy, but the entire last two movies simply cannot have happened based on how they presented it. I see no way of explaining this one, but would love to hear your theories!