People hate plot holes in movies. At least, that's what they'll tell you. But sometimes, if a movie is awesome enough, people will overlook even the most retarded gaps in reason and logic.
At least, until some asshole on the Internet points them out and makes a big list of them. Enjoy:
8Back to the Future
Marty McFly goes back in time, helps his parents get together, invents rock and roll...
...and everyone promptly forgets he was ever there the minute he leaves.
Nobody notices that a famous clothing brand is later named after him, nobody notices that Chuck Berry releases a song that sounds pretty similar to the one he played at the big dance, and most importantly, nobody bats an eyelid when his Mom has a kid who looks exactly like him.
Now we don't claim to know exactly what first enters the mind of a married man when his wife births a child who looks identical to their old high school boyfriend, but we're guessing it's not "time travel conspiracy." Old George was either the most oblivious, forgiving man on earth, or there were some secret resentment beatings in the McFly household.
Even more disturbing, what must his Mom have thought? The only explanation we can see making sense from her point of view is that Marty was Satan (he did invent rock and roll after all) and the whole thing's some kind of demon spawn Rosemary's Baby type deal. And no one should ever be in a position where the most plausible explanation for their situation implies that they fucked Satan.
This was the most sinister looking picture of Michael J Fox we could find.
Plus, think how chilling Marty's final remark on stage becomes given this context: "I guess you're not ready for that yet... but your kids are gonna love it."