Marty, whatever happens, you must not let your other self see you! The consequences could be disastrous.
5 seconds sound clip from the Back to the Future Part II movie soundboard.
You can hear this line at 01:13:34.544 in the Blu-ray version of the movie.
Quote context
[...]
- Marty, listen, we may have to abort this entire plan. It's getting much too dangerous.
- Don't worry. The book is on Biff's dashboard. I'll grab it as soon as we get to the school.
- Marty, you must be extremely careful not to run into your other self.
- Yes.
- Remember, your mother is at that exact same dance with you. Yeah.
- Right. This could get heavy, Doc.
- Excuse me, sir. Yes, you with the hat.
- Who, me?
- Yes.
- Be a pal and hand me a five-eighths inch wrench out of that toolbox.
- Five-eighths? Don't you mean three-quarters?
- I presume you're conducting some sort of weather experiment.
[...]