A jury is said to be hung when it is unable to reach an agreement on a verdict. In cases where a unanimous verdict is required, this can be caused by a single dissenting juror. Usually, the judge will do everything in his or her power to avoid a hung jury, and may send the jurors back to the jury room in the hopes of reaching a decision. However, when a hung jury is unavoidable, the judge must declare a mistrial.
A mistrial may be declared for reasons other than a hung jury; for instance, the death or severe illness of an attorney or juror, a fundamental error in the jury selection process or misconduct by a juror all may be grounds for a mistrial.
A prejudicial error in prosecution that unfairly influences the jury’s opinion of the defendant and which may not be reconciled by instructions to the jury may be cause for a mistrial as well.
Judges often are hesitant to declare a mistrial, as retrying a case is costly—in time, money and resources. In the case of a hung jury, the judge may send jurors back to the jury room in the hopes of reaching a unanimous (or majority, if applicable) verdict.
In cases of a fundamental error or other extenuating circumstances, judges still must exercise great caution when ruling a mistrial.