Elizabeth Proctor, the wife of John Proctor in Arthur Miller's The Crucible, is brought into court in order to testify regarding accusations John has made (Abigail being a whore). During John's testimony, he states that Elizabeth cannot lie ("In her life, sir, she have never lied. There are them that cannot sing, and them that cannot weep—my wife cannot lie.").
Wanting to break John, the courts bring Elizabeth in. They wish to settle the case against John once and for all. They do not allow Elizabeth to look at John at all (in case he tries to tell her something). Unbeknownst to Elizabeth, John has admitted to his affair with Abigail. Wanting to save him, Elizabeth replies that John is a "goodly man" and that he had never "turned" from her.
Finding Elizabeth's testimony to be true (although John admits she only lied to save him), the court finds him guilty of lying. The irony lies in the fact that, although Elizabeth thought she was saving him, her testimony sent John to the gallows instead.