Elizabeth's lie in the courtroom is perhaps the most dramatic moment in Act III of The Crucible; but the crisis, it seems to me, happens before that. John has come to court to try to persuade them that the girls were lying. He tried several lesser arguments--no doubt hoping he could make them see the truth without any great risk to himself--but to no avail. It is only when Proctor admits to his adulterous relationship with Abigail, the ringleader of the accusing girls, that the judges take notice.
This is a pivotal moment, a crisis moment, for two reasons: 1) it does capture the attention of the judges in a way nothing else did, and 2) by admitting to the sin/crime of adultery, John Proctor has signed his own death warrant. Adultery was a capital offense, and he knew it. In this case, the witchcraft accusations supercede the adultery, but he knows his life will never be the same after this admission. John Proctor's admission, I think, is the moment of crisis in this act. Elizabeth's well intentioned lie simply seals their fates.