Smith v. Massachusetts, Supreme Court of the United States, February 22, 2005 [ruling 5-4 that a trial judge's reversal of a mid-trial ruling to dismiss a firearm charge constituted double jeopardy]. Excerpt from the majority opinion by Justice Scalia:

The Double Jeopardy Clause's guarantee cannot be allowed to become a potential snare for those who reasonably rely upon it.If,after a facially unqualified mid-trial dismissal of one count,the trial has proceeded to the defendant 's introduction of evidence,the acquittal must be treated as final, unless the availability of reconsideration has been plainly established by pre-existing rule or case authority expressly applicable to midtrial rulings on the sufficiency of the evidence.That requirement was not met here.

Read the full text of the opinion here [PDF]. Reported in JURIST's Paper Chase here.