The complete Ghostbusters script:
EGON: Don't cross the streams.
EGON: It would be bad.
PETER: I'm fuzzy on the whole good-bad thing. What do you mean, bad?
EGON: Try to imagine all life as you know it stopping instantaneously and every molecule in your body exploding at the speed of light.
RAY: Total protonic reversal.
PETER: All right, that's bad, okay. Important safety tip, don't cross the streams. Thanks, Egon.
Don't cross the streams. It's a metaphor for life. (And for cvs, but that's another story entirely.)