Don't Not Do It!

Don't do anything that effects anything, unless it turns out you were supposed to do it. Then, for the love of God, don't not do it!

Source: Professor Farnsworth from the Futurama episode Roswell That Ends Well.

Explanation: The professor is warning his crew that they have gone back in time and they should not under any circumstances (unless they should) interfere with the past. Fry naturally goes on to kill his grandfather and then becomes his own grandpa.

Here's a clip including the ever popular "Truman Hitting on Zoidberg" scene:



Special Blog Bonus: It has nothing to do with Futurama, but here's the movie poster for I Am LEGO: