A writ of coram nobis is an order by an appeals court to a lower court to consider facts not on the trial record which might have changed the outcome of the lower court case if known at the time of trial. Coram nobis is a Latin term meaning the "error before us."
In deciding whether to grant the writ, courts have used a three-part test: a petitioner must
1. explain her failure to seek relief from judgment earlier,
2. demonstrate continuing collateral consequences from the conviction, and
3. prove that the error is fundamental to the validity of the judgment.
An example of when it might be used includes prosecutorial misconduct hiding exculpatory evidence from the defendant. A writ of coram nobis is issued once the petitioner is no longer in custody. Its legal effect is to vacate the underlying conviction. A petition for a writ of error coram nobis is brought to the court that convicted and sentenced the defendant.
Coram nobis is limited to cases in which a "fundamental error" or "manifest injustice" has been committed. A high burden of proof is required. It cannot be used to reopen and reargue points of law the courts have decided, but only to raise errors of fact that were knowingly withheld by the prosecutor from judges and defendants. A writ of error coram nobis is an extraordinarily rare remedy, known more for its denial than its approval. It is distinguished from a writ of error coram vobis which brings before the court certain mistakes of fact not put in issue or passed upon,such as the death of a party, coverture, infancy, error in process, or mistake of the clerk.
Although the Federal Rules of Civil Procedure expressly abolished the use of coram nobis in civil cases in the United States, see Fed. R. Crim. P. 60(b), the issue of the writ's availability to correct fundamental errors in criminal cases remained uncertain for many years. In United States v. Morgan, the Supreme Court resolved this question, holding that coram nobis was still available in federal court for criminal cases. See 346 U.S. 502, 512 (1954).