Doc: They're taking her home, to your future home! We'll arrive shortly thereafter, get her out of there and go back to 1985.
Marty McFly: You mean, I'm gonna see where I live? I'm gonna see myself as an old man?
Doc: No, no, no Marty, that could result in a-
[gasps]
Doc: Great Scott! Jennifer could conceivably encounter her future self! The consequences of that could be disastrous!
Marty McFly: Doc, what do you mean?
Doc: I foresee two possibilities. One, coming face to face with herself 30 years older would put her into shock and she'd simply pass out. Or two, the encounter could create a time paradox, the results of which could cause a chain reaction that would unravel the very fabric of the space time continuum, and destroy the entire universe! Granted, that's a worse case scenario. The destruction might in fact be very localized, limited to merely our own galaxy.
Marty McFly: Well, that's a relief.