This might be a bit too coarse, but here we go:
"Assumptions is the mother of all fuckups!"
Heard in a movie. Think it was one of the "Alien" ones, but I'm not sure.
edit: Umm, no. It's apparantly been used in "Under Siege 2" (google is my friend). But thinking about it, I distinctly remember John Travolta using this line in "Broken Arrow" and that is where I get it from. Ahh, the Irony, JT using what could only be defined as a skeptical slogan