The law of self-defense.
Self-defense is an affirmative defense, so the defendant has the burden of producing evidence: He must put on some evidence from which a jury can find self-defense. But then the burden of proof returns to the prosecution, which must disprove self-defense beyond a reasonable doubt.
It was not always thus. The English common law rule at the time of the Framing was that the defendant must prove self-defense by a preponderance of the evidence, and the Supreme Court has held (Martin v. Ohio (1987)) that placing this burden on the accused would be constitutional. But even by then, “all but two of the States, Ohio and South Carolina, ha[d] abandoned the common-law rule,” and they have since changed their rule by statute. (The only exception I know of is the view of some Louisiana appellate courts in non-homicide cases (see State v. Satterfield (La. Ct. App. 2021)), which still require the defendant to prove self-defense by a preponderance of the evidence; Louisiana follows the unanimous beyond-a-reasonable-doubt rule as to self-defense in homicide cases.)