In Act III of "The Crucible," what can Mary Warren not do when requested, and what point does Judge Hathorne make about it?

When asked to pretend to faint, Mary is unable to do so. The judge makes the point that if she cannot pretend to faint at that point then perhaps she did not pretend to faint before. This casts a lot of doubt on Mary's claim that Abigail and the rest of the girls are also pretending. Mary's credibility is also destroyed and eventually she rejoins Abigail in making accusations against John Proctor.