We discussed in Civ Pro the difference between an element of a claim and an affirmative defense. The distinction takes on added importance in criminal trials, as the distinction triggers two concerns: 1) who bears the initial burden of production and 2) even if the defendant bears the burden of production, who bears the burden of persuasion, as criminal law often disaggregates them.
So consider this Tenth Circuit case. Under 18 U.S.C. § 1152, federal criminal law does not apply to "offenses committed by one Indian against the person or property of another Indian." So if the victim of a crime in "Indian country" (as the statute calls it) is an Indian, the defendant cannot be convicted if he also is an Indian. So, is Indian status an element (government bears the burden of production and persuasion to show defendant is not Indian) or an affirmative defense (defendant bears the burden of production that he is an Indian). And if the latter, who bears the burden of persuasion.
The Tenth Circuit held that it is an affirmative defense and that defendant bears the burden of production and persuasion (show Indian by preponderance). How the court got there is worth a read.