Marty: "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... (gasps) Great Scott! Jennifer could conceivably encounter her future self. The consequences of that could be disastrous."
Marty: "Doc, what do you mean?"
Doc: "I forsee 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 worst case scenario. The destruction might, in fact, be localized, limited merely to our own galaxy."
Marty: "Well, that's a relief."