To be convicted of being a felon-in-possession of a firearm, the government must prove that the defendant knew he possessed a firearm and knew he was a felon at that time. The case of Greer v. United States involved two different situations, one where a defendant failed to request a jury instruction requiring the jury to find that he knew he was a felon at the time he possessed the firearm, and the other where the defendant agreed to a plea deal without knowing that the jury would have to find such knowledge. The Eleventh Circuit held that the failure to instruct the jury was not plan error and thus was upheld, while the Fourth Circuit held that the plea deal was a structural error that required reversal.
The Court, in a unanimous opinion by Justice Kavanaugh, held that while both errors were plain, the substantial rights of the defendants were not violated because the outcome would not have been different for either but for the error. A defendant challenging such errors must be able to prove that he did not in fact know that he was a felon at the time he possessed a firearm—something neither defendant this case did. The Court also rejected the argument that the Fourth Circuit’s finding of a structural error required relief. However, Justice Sotomayor lodged a solo partial dissent suggesting that the defendant who made the plea deal should have been allowed to make his evidentiary case before the lower courts.
A link to the opinion is here: https://www.supremecourt.gov/opinions/20pdf/19-8709_n7io.pdf