Looking back on her role in the Salem witch trials, Mary Warren would have to reflect on 1) the question of how much influence she did or did not have on the outcome of the trials and 2) her failure when tested.
Mary Warren has a chance to invalidate the trials when she comes to court with John Proctor. There is a real question as to how much effect she could have on the trial's outcome given Abigail's strength in the court and Danforth's persistence concerning the convictions already decreed. However, Mary Warren would be likely to reflect ruefully and regretfully on losing her chance to save lives.
She was somewhat close to John Proctor who dies in the end of the play, as well as Elizabeth who is also sentenced to death. These deaths in particular would be predicted to bother her for the rest of her life.
When Mary Warren is given the opportunity to tell the truth, she makes an effort but fails utterly to convince the court and to stand by her own truth.
This failure, perhaps even more than the actual loss of life resulting form it, should bother her later because this failure makes her complicit in the proceedings. This moment, so regrettable a failure, is the poignant episode for her in the trial. Her role is one of weakness and this fact certainly play in her mind on recollecting the trial later. Though she is coerced, she is nonetheless guilty of incapacity, weakness and a lack of integrity.