In "The Crucible" Mary Warren's testimony is destroyed in the end because she cannot do something. What? How does she explain the problem?

Mary Warren comes the courts in act three to testify that she and the other girls had been faking their testimonies the entire time. They were pretending to faint and be afflicted by witches.  Mary is scared to give this testimony, because, as she says at the end of act two, "they'll turn on me," referring to the other girls.  They will turn on her and accuse her of being a witch if she dares speak up about their theatrics.  But, Mary does go to court, and despite her fear, tries to testify.  However, the judges, and Parris, are skeptical.  They don't get how they girls could be faking.  They say that she herself would faint dead away and be cold to the touch--how did she do that?  She tells them it "were all pretense," and so they say, well, pretend now.  If you can pretend in court, pretend now.  Mary tries, really hard, but can't do it.  You might think it's easy to fake a faint, but she is in a highly pressured situation, with everyone staring at her.  Her life is potentially on the line, she's nervous, and can't do it.  She says that it was easy in court because all of the other girls were screaming and fainting, and she caught the hysteria of it all and went along with it, almost believing it herself. If you have ever started screaming in a crowd of people, simply because others were screaming, or if you've been to a concert and gotten carried away with the crowd, you know what she felt in the courtroom.

So, because she couldn't fake a faint while alone, the judges conclude she must be lying.  Abby helps them along by pulling her own stunt, pretending that Mary is a bird come to tear her eyes out.  In the end, Mary's testimony is weakened at that point, and eventually, she caves, turning on John, and going back to her friends.  I hope that helps.  Good luck.