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1.
Don't fuck with cats or we'll find you.
"hey /b/, ima kill these cats, will post pics"

"rule zero. you'll be reported to the FBI, faggot"
by yxxoB December 28, 2009
 
2.
When you're trying to figure out whether doing something is a good idea or not, there are various rules that come in to play, such as Have Fun, Do No Harm, Have Fun (again), What Happens in Vegas, etc.

Rule Zero, the rule that always comes before all the others, the most important, basic rule of doing anything is: "Don't be on fire."
- I can't get this bbq to light.
- Here, use some petrol.
- I am not breaking Rule Zero to cook a burger.

--
- Hey, let's see what happens if I fill my foreskin with lighter fluid and light it.
- I am invoking Rule... wait... yeah, go ahead, that'll be funny.
by tbrd June 15, 2010
 
3.
The customer always lies.
The customer said this happened. Dude...Remember Rule Zero!
by Tech Support Rep January 18, 2012