It's an affirmative defense, so the standard of proof would be mere preponderance of evidence, i.e., a reasonable person would conclude, based on the facts asserted by the defense (along with proffered evidence), that it's more likely the case than not. I think such things as her voice having been recorded saying, right before the shooting, "I'm going to Taser you," while holding the Glock, and then her voice, after the shooting, expressing surprise, stating "I just shot him," would satisfy the standard.
An example of a case where the mistake of fact was not found to be an adequate defense can be found in Allen v. State, 290 Ga. 743, (2012). There the defendant believed that the victim was not in the trajectory of the bullet fired at a third party. The Court ruled that his belief did not constitute the type of mistake of fact that would serve as a defense to malice murder or other crimes."
I got banned on another web site for a debate that happened on this site. That's a first
