What is the defense of mistake of fact?

An accused person may claim that she was mistaken about an essential fact and therefore did not have the criminal intent necessary to commit the crime.

For example, a person who really believes she was carrying baby powder does not have the criminal intent necessary to be guilty of the offense of possessing cocaine.  Or, to cite another example, an individual who is conspiring to accomplish one goal is not guilty if the conspiracy she was charged with alleges that she was conspiring to accomplish a different goal.