Doc: The consequences of that could be disastrous.
Marty: Doc what do you mean?
Doc: I foresee two possibilities. One: coming face to face with herself thirty years older would put her into shock and she'd simply pass out. Or two, the encounter could create a time paradox, the result of which could cause a chain reaction that would unravel the very fabric of the space-time continuum and destroy the entire universe! Grant it, that's worst-case scenario, the destruction might in fact be very localized, limited to merely our own galaxy.
Marty: Well, that's a relief.
— Back to the Future Part II