The irony in her lie is that she went against her morals to protect John's honor in hopes of salvaging him, but instead she doomed them both.
Elizabeth, not knowing what went on in the courtroom, tries to protect her husbands honor but ironically condemns him as well as herself to death. The accusations in Salem have gotten dangerously out of hand and must be stopped. The misfortune of John Proctor and his wife just goes to show you that in the end the truth really will set you free.
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