Doc: The encounter could create a time paradox, the result of which could cause a chain reaction that would unravel the very fabric of the spacetime continuum and destroy the entire universe! Granted, that's a worst-case scenario. The destruction might in fact be very localised, limited to merely our own galaxy.

Marty: Well, that's a relief.

Good thing Biff Tanner becoming president was a silly fictional alternate reality. Phew.