Mary Warren, in Act III, after she submits testimony to the court, telling the truth for the first time, she is unsuccessful. The court does not believe her, the judge warns her that if she has lied to the court at all, she will hang.
Mary Warren gets scared, terrified as a matter of fact, she turns and accuses John Proctor of being in league with the Devil and of trying to get her to join him. She lies.
She continues to lie, she claims that he has threatened her, she says. She lies and tells the court that he has threatened to kill her if she did not lie in court to get Elizabeth Proctor free.
Mary Warren dooms John Proctor. She lies about him to save herself. She is responsible for getting him arrested and ultimately executed.