"The 'Sports Almanac'. Son of a bitch stole my idea! He must have been listening when I... It's my fault! The whole thing's my fault. If I hadn't have boughten that damn book, then none of this would have ever happened."
"Well, forget it Marty. That's all in the past."
"You mean the future."
"Whatever! It demonstrates precisely how time travel can be mis-used, and why the time machine must be destroyed, after we straighten all of this out."
"They're taking her home, to your future home! We'll arrive shortly thereafter, get her out of there and go back to 1985."
"You mean, I'm going to see where I live? I'm gonna see myself as an old man?"
"No, no, no Marty, that could result in a... Great scott! 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 would simply pass out. Or two, the encounter could create a time paradox, the results of which could start 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 our own galaxy."
"Well, that's a relief."