In Henderson v. United States the court had to decide whether an error was “plain” during the appellate proceedings, but wasn’t at the time of trial – well at least was “unsettled” at the time of trial.
Breyer writes for a majority which did not include Scalia, Thomas, or Alito.
I’m not sure about this opinion. If there is an issue and it is “unsettled” doesn’t the defense counsel know about it at the time of trial. Therefore an effective defense counsel should raise the issue and point out the law is unsettled. The error is preserved and the judge is given the opportunity to rule in the defense favor.