— Doc! Doc, come in!
— Marty, come in.
— Listen, Biff's guys chased me into the gym and they're gonna jump... me.
— Then get out of there!
— No, Doc, not me. The other me. The one that's up on stage playing «Johnny B. Goode».
— Great Scott! Your other self will miss the lightning bolt. You won't get back to the future and we'll have a major paradox!
— Wait. A paradox? You mean one of those things that can destroy the universe?
— Precisely.

— Jennifer could conceivably encounter... her future self. The consequences of that could be disastrous.
— Doc, what do you mean?
— 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 worst-case scenario. The destruction might, in fact, be very localized, limited to merely our own galaxy.
— Well, that's a relief.