Smith v. Massachusetts
After the prosecution has finished presenting evidence - including statements along the lines of "He shot me with a pistol - either a .38 or.32" - the defense moves to have an unlawful possession charge found not guilty because one of the necessary elements of the charge has not been proven: that the firearm is less than 16 inches.
While I'd have thought it fairly common knowledge that a .38 or a .32 pistol is less than 16" apparently it wasn't and the judge approves of the defendant's motion. Perhaps he understood the law to require actual introduction of the length into evidence and did not believe an inference based on common knowledge to be sufficient. In any event it is entered into the record that the motion is granted.
At the close of all evidence (there were other charges) the prosecutor presents the judge with case law that convinces him that he was wrong in his earlier decision. Before the jury is given instructions or closing arguments the judge reinstates the charge (jury never knew it was gone) and defendant is found guilty.
Justice Scalia, writing for the majority, finds that this violates double jeopardy. The mid-trial acceptance of the defendant's motion is an acquital and therefore reinstating the charge after the defendant might have introduced prejudicial evidence he would never have introduced had the charge remained is a "snare" which defendants ought not to have to face.
If a State's statutes, rules of court, announced common law, or exercise of the State supreme court's "supervisory power" (huh? Wouldn't that be the rules?) allow reinstatement of a charge then double jeopardy does not occur because . . . Well, okay the decision doesn't really say why. I'd guess it would be because the defendant had fair warning of the rules of the game.
Nevertheless, when the Massachusetts appellate court laid down a similar rule it went too far because it didn't have this issue settled prior to this case. Interesting. Perhaps if the Massachusetts Supreme Court had taken the case it could have exercised its "supervisory powers" and declared this procedure valid and been approved by the Federal Supreme Court. However, all the same arguments regarding prejudice against the defendant would still apply so that seems unlikely.
Basically, it looks like the Supreme Court has shut down common law in this area. The question becomes whether this will lead to any changes in the statutes or rules of court or if it will become the accepted standard?