"This case demonstrates that the threat to individual freedom from reprosecutions that favor States and unfairly rescue them from weak cases...

"... has not waned with time. Only this Court’s vigilance has." 

So ends the dissenting opinion in Blueford v. Arkansas, written Justice Sotomayor and joined by Justices Ginsburg and Kagan. It's quite an interesting double jeopardy problem, where a mistrial was declared after a report from the jury that there had been a unanimous vote of not guilty on the capital murder charge and the first degree murder charge.

The jury got hung up at the manslaughter charge. As Chief Justice John Roberts wrote for the majority, the jurors were instructed to decide one charge at a time and only to move on to consider the next, lesser charge after they'd found the defendant not guilty on the more serious charge. Roberts writes that despite the report from the jury, mid-deliberation, that revealed that there had been a unanimous not guilty vote on the 2 highest charges, he hadn't been "actually acquitted" of them.

The foreperson’s report was not a final resolution of anything. When the foreperson told the court how the jury had voted on each offense, the jury’s deliberations had not yet concluded. The jurors in fact went back to the jury room to deliberate further, even after the foreperson had delivered her report. When they emerged a half hour later, the foreperson stated only that they were unable to reach a verdict. She gave no indication whether it was still the case that all 12 jurors believed Blueford was not guilty of capital or first-degree murder, that 9 of them believed he was guilty of manslaughter, or that a vote had not been taken on negligent homicide. The fact that deliberations continued after the report deprives that report of the finality necessary to constitute an acquittal on the murder offenses....

But even if we assume that the instructions required a unanimous vote before the jury could consider a lesser offense—as the State assumes for purposes of this case, see Brief for Respondent 25, n. 3—nothing in the instructions prohibited the jury from reconsidering such a vote. The instructions said simply, “If you have a reasonable doubt of the defendant’s guilt on the charge of [the greater offense], you will [then] consider the charge of [the lesser offense].” App. 51–52. The jurors were never told that once they had a reasonable doubt, they could not rethink the issue....