Under Brady v. Maryland and other cases prosecutors are required to disclose exculpatory and impeaching evidence. Prosecutors cannot knowingly use false evidence to obtain a criminal conviction and must correct false evidence presented.
In Phillips v. Ornoski, a capital habeas corpus petition, the Ninth Circuit vacated a special circumstances finding that led to petitioner's death sentence because the prosecutor concealed from the defense and the jury that the star witness had been given leniency in exchange for testifying.
"Because Phillips was denied a fair opportunity
to impeach the credibility of a witness whose testimony was essential to the jury's special circumstance finding," the Court held, "we conclude that there is 'a reasonable likelihood that the false testimony could have affected the judgment of the jury,' with respect to the finding that [the]
murder occurred during the commission of a robbery."