Texas Penal Code - PENAL § 8.02. Mistake of Fact

(a) It is a defense to prosecution that the actor through mistake formed a reasonable belief about a matter of fact if his mistaken belief negated the kind of culpability required for commission of the offense.

(b) Although an actor's mistake of fact may constitute a defense to the offense charged, he may nevertheless be convicted of any lesser included offense of which he would be guilty if the fact were as he believed.