Admittedly, this one has limited relevance to cops (since most of us don't really deal with affirmative defenses and their impact on other charges). But I still thought it was interesting, because it's a murder case and I remember when it was on the news.
Doubleday robbed a convenience store, and during the robbery he shot and killed the clerk. At the trial, he claimed the affirmative defense of duress, testifying that members of the Gallant Knights Insane had assaulted him and threatened to kill him and his family if he didn't do a robbery to pay off a drug debt. A GKI member then drove him to the convenience store, where he accidentally shot the clerk during the robbery.
He was arrested and charged with (among other things) attempted aggravated robbery and felony murder. The felony murder statute essentially makes it first degree murder if someone dies (even accidentally) during the commission of a specific list of felonies (including robbery). Duress is an affirmative defense which states that if someone commits a crime (other than a class one felony, such as first degree murder), but they do so because someone else is making violent threats that a reasonable person would be unable to resist, then they can't be convicted.
Anyway, the jury bought the duress offense, and found Doubleday not guilty of the attempt robbery by virtue of the threats that the GKI had made. Still, the jury found Doubleday guilty of murder, and he was sentenced to life. He appealed, arguing that since he was not convicted of the underlying felony that allowed for felony murder, the murder conviction should also be reversed.
The Court of Appeals pointed out that felony murder makes it a crime to accidentally kill someone while committing an underlying felony, without actually requiring that the suspect be convicted of that felony (or even charged with it). Since the prosecution proved all the elements of the attempt robbery, and he was acquitted solely on the basis of an affirmative defense, the felony murder conviction stands.