It's not wise to over-think movies, especially the kinds of summer junk-food movies that seem to lodge themselves so firmly in our hearts at a young age. These are, after all, transient entertainments, with just the right mix of internal consistency, panache and charm to make a big impression when we first see them. Unfortunately, with the help of VHS and DVD, we end up watching these films over and over again, and around the three hundredth viewing you start to see the strings.
So, for example: The Empire Strikes Back is easily the best entry in the whole Star Wars franchise. But it occurs to me that the time Luke Skywalker spends on Dagobah, learning (we are later told) almost everything he needs to know to become a Jedi, is about the same time as Han and Leia spend fleeing from Hoth to Cloud City. He accumulates generations of ancient wisdom and gnarly paranormal powers during what is basically an extended car chase. That's like going to a Tony Robbins seminar and coming out with a black belt in every martial art ever invented, plus a doctorate in particle physics.
Another thing that's troubled me lately (if by "troubled" we mean "occurred to me as I lie here waiting for my flu to blow over") is Marty McFly's future in Back To The Future. If you'll recall, the most important thing that happens in that trilogy is that he goes back to 1955 and changes the past, so that when he gets back to 1985, his life is way better. His dad's not a coward anymore, his mom's not an alcoholic anymore, his brother's not a loser, they have more money etc.
Which is great. But the thing is, now he's going to spend the rest of his life with a completely different set of memories of his family than they have. (That's how it works in BTTF. Matchbooks and photos get rewritten when the time they came from changes, but your memory of things stays the same. I know--it only occurs to you after the 600th viewing. Or when some jerk with a blog points it out.)
But isn't this going to make him kind of crazy? I mean, his family are now completely different people, and their experiences over the last 17 years will bear little ressemblance to what he remembers. A lot of the things that they've done together (from Marty's perspective) now never happened. No one will remember the trip they took to Six Flags because, in this new/improved timeline, George McFly made enough money to take them to Disneyworld. Every Christmas and Thanksgiving for the rest of his life, he's going to sit there while everyone talks about old times and he won't know what the hell they're talking about. Even his own experiences with the rest of the world will probably have been completely different.
The more I think about it, the more awful it sounds--like your family has been replaced by replicants and your own life lived by a stranger. You'd feel like an impostor, or like everyone else is. If you ever disagreed with anyone about the fact of a past event, you wouldn't have a leg to stand on. Your confidence in your ability to remember anything would be permanently shaken, more and more as time goes on, as memory itself gets fuzzier and timelines confuse themselves without any help from a flux capacitor.