A courtroom trial that has been terminated prior to its normal conclusion. A mistrial has no legal effect and is considered an invalid or nugatory trial. It differs from a "new trial," which recognizes that a trial was completed but was set aside so that the issues could be tried again.
A judge may declare a mistrial for several reasons, including lack of jurisdiction, incorrect jury selection, or a deadlocked, or hung, jury. A deadlocked jury—where the jurors cannot agree over the defendant's guilt or innocence—is a common reason for declaring a mistrial. Extraordinary circumstances, such as death or illness of a necessary juror or an attorney, may also result in a mistrial. A mistrial may also result from a fundamental error so prejudicial to the defendant that it cannot be cured by appropriate instructions to the jury, such as improper remarks made during the prosecution's summation.
In determining whether to declare a mistrial, the court must decide whether the error is so prejudicial and fundamental that expenditure of further time and expense would be wasteful, if not futile. Although the judge has the power to declare a mistrial and discharge a jury, this power should be "exercised with great care and only in cases of absolute necessity" (Salvatore v. State of Florida, 366 So. 2d 745 [Fla. 1978], cert. denied, 444 U.S. 885, 100 S. Ct. 177, 62 L. Ed. 2d 115 ).
For example, in Ferguson v. State, 417 So. 2d 639 (Fla. 1982), the defendant moved for a mistrial because of an allegedly improper comment made by the prosecution during closing argument. The prosecution stated that not only was defense counsel asking the jury to find a scapegoat for the defendant's guilt, he was also putting the blame on someone who had already been found guilty. The appellate court found that the lower court had properly denied the motion for a mistrial because the prosecutor's comment fell within the bounds of "fair reply."
A mistrial in a criminal prosecution may prevent retrial under the DOUBLE JEOPARDY provision of the FIFTH AMENDMENT, which prohibits an individual from being tried twice for the same offense, unless required by the interests of justice and depending on which party moved for the mistrial. Typically, there is no bar to a retrial if the defendant requests or consents to a mistrial. A retrial may be barred if the court grants a mistrial without the defendant's consent, or over his objection. If the mistrial results from judicial or prosecutorial misconduct, a retrial will be barred. In United States v. Jorn, 400 U.S. 470, 91 S. Ct. 547, 27 L. Ed. 2d 543 (1971), the Supreme Court held that reprosecuting the defendant would constitute double jeopardy because the judge had abused his discretion in declaring a mistrial. On his own motion, the judge had declared a mistrial to enable government witnesses to consult with their own attorneys.