Counter Example

In logic, one counter-example completely disproves an assertion of fact.  For example, the National Rifle Association has made the statement:

"The only thing that stops a bad guy with a gun, is a good guy with a gun."

As the tragic shootings at the Washington Navy Yard show, this is false.  In this case the killer shot an armed security guard - kind of the definition of  'a good guy with a gun'.  Not only that, he took the guard's own gun.

So  to recap, the good guy with the gun did *not* stop the bad guy with a gun. Even worse, the good guy's gun was used by the bad guy for bad things.  The good guy did not stop the bad guy and, in fact, his presence with that gun contributed to the violence.