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Resetting timelines from before conception
Tim's father warns him about changing anything before a conception, since it is impossible to duplicate the timing exactly to get the same sperm to impregnate the egg. However, Tim IS able to reset his change after he accidentally erases his first daughter. Okay, this is explainable with the theory that he can undo a change entirely if he so chooses. But if that's a possibility, then why does the arrival of his third child mean he can never travel back to see his father again? Couldn't he just do a reset like he did before?
- This is actually addressed in the ending. Theoretically, if he travels back to a totally non-critical point in time, or changes absolutely nothing, he can take short trips back. On an emotional level, however, he would be constantly going back to a time when his father is dying from a slow, incurable disease, and would likely never gain the closure he needs. This is in-keeping with his ethos at the end of the film, where he no longer time travels at all, and instead lives like he is reliving every day for the joy of it.
Memories after going Back To The Future
Tim's sister experiences a rush of memories of her new life after Tim takes her back (and forward again) with him. How come Tim doesn't experience the same thing? He is surprised to have a son. It seems this is just another arbitrary wrinkle in the time travel ability, which is a bit unsatisfying.
- The way I interpreted this is that you do have the new memories if you stop and think about them, but they wouldn't necessarily be front and center if you didn't. So Kit Kat stops to mentally inventory what's now different about her life, but Tim doesn't do the same for himself, because he's so focused on his sister. It's sort of like when you move to a new place in the same city, but you accidentally start driving home to your old place for the first few weeks—if you stop and think about it, you know that you live in a different place now, but your mind is still on autopilot for a while.
Time travel mechanics
Many of the standard time travel specifics are glossed over. For instance Tim, at least some of the time, arrives in the past in the same location that he left from. Does this mean his past body teleports from where it was in the original timeline? Does he always make sure to pick a moment in the past where he was unobserved, so that this teleportation goes unnoticed? What if the location was occupied at his destination time? Does the timeline automatically adjust his/others' actions so this is never a problem? There's also the question of what happens to past-him if he chooses to go back to the future: Does past-him lose his memories from the future, and does he get returned to his previous location if he was "teleported"? If past selves revert to past memories, how do they account for their actions while "possessed" by future-selves? Why did Kit Kat think she'd punched Jimmy? All of these things are just assumed to not be important to the story, but a certain type of person wonders about such things...
- The first time Tim time travels he starts by standing in his wardrobe and ends up in the same wardrobe the day before despite not having been in there before.
