At times, a trial cannot continue, due to an irrevocable error which corrupts the process to the point that a fair and impartial conclusion cannot be reached. Impartiality is the premise of any trial and if it is impossible to achieve, a mistrial must be declared. Generally, such an error prejudices one party so badly that the jury cannot reach a verdict, or the judge cannot ensure that the verdict would be a fair one.
In some cases, a mistrial is requested due to seemingly unfair comments made or inadmissible evidence being presented in opening or closing remarks. If jury instructions cannot repair the damage, the judge may have to call a mistrial. It is difficult to expect a jury to simply ignore such statements, and to then proceed in an objective manner as if they had not been made.
Unexpected events, especially of the catastrophic nature, can also result in mistrial. The death of an attorney, judge, juror, or necessary witness may necessitate it. An attorney may file a motion for a mistrial. Grounds for mistrial must be shown, reflecting that the situation cannot reasonably be restored to an impartial proceeding by other means.
Since cases with glaring errors will generally be overturned upon appeal anyway, a judge will likely grant the motion. In some cases, the judge will call a mistrial without a motion being filed, if the circumstances overwhelmingly indicate that either a unanimous or an objective verdict is impossible.
Even when a mistrial is declared, it does not necessarily mean that the case is over. It may be dropped or a new trial may be ordered. The judge will declare it to be with or without prejudice. If it is declared with prejudice it means the case cannot be retried.
A mistrial with prejudice will occur in cases that involve prosecutorial misconduct or judicial misconduct. This is possible in high profile cases when it appears that assembling an impartial jury is not possible. It may also be necessary if the available evidence is unduly prejudicial and must be excluded. If the prosecution cannot make its case without that evidence, there is no sense in retrying it.
The term without prejudice means the case may be retried at a later date. After declaring a mistrial, the judge may order a new trial. The decision to go forward or to drop the case completely is sometimes left up to the prosecution, based on its ability to effectively and fairly retry the case. If a new trial is ordered, it will start from the beginning and require the assembling of a new jury.