Doc: Great Scott! Jennifer could conceivably encounter her future self! 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 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 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
I saw the back of my own head! The actual back of my actual head! No-one's done that since the Spanish Inquisition!