Great Scott! Your other self will miss the lightning bolt at the clock tower, you won't get back to the future, and we'll have a major paradox!
5 seconds sound clip from the Back to the Future Part II movie soundboard.
You can hear this line at 01:24:51.887 in the Blu-ray version of the movie.
Quote context
[...]
- Listen, Biff's guys chased me into the gym, and they're going to jump... me!
- No, Doc, not me. The other me. The one that's up on stage playing Johnny B. Goode.
- Wait, wait, wait. A paradox? You mean one of those things that can destroy the universe?
- 10-4.
- Who?
- Who?
- The guy with the hat, where is he?
[...]