Friar Lawrence made the most danger mistake, when he trusted an unsettled teenage girl, whit a deathly poison. Here the Friar shows his irresponsibility by saying, "If... thou hast the strength of will to slay thyself... take thou this vial... no warmth, no breath shall testify thou livest;" (4.1.72, 93, 98) . Friar Lawrence's idea for Juliet is very risky and he should have known better than to try it. Because of what happens, everyone thinks Juliet is dead and shortly thereafter, she is buried alive. This quote shows the true meaning of responsibility, because when he says this, her life rests in his hands. Capulet is even foiled by the plan, because when he says her faking her death, he said, "Death lies on her like an untimely frost." (4.5.28). The fake death has fooled Capulet, Juliet's father, and the rest of the family. Had Friar Lawrence not have given Juliet the poison, she would have never been put in the position that she was in, which eventually leads to her death.
Trusting the Friar John to send the latter to Romeo without even tailing him how critical is the letter is the Friar Lawrence’s next big mistake. The mistake of him sending someone else to do it was inexcusable; a matter as important as faking death should be dealt with personally. Had Friar Lawrence have personally delivered the letter, the plan might have gone smoothly. Friar John shows his incompetence in the fifth act when he says "I could not send it - here it is again -" (5.2.14).