Friar Laurence's plan starts going wrong when he decides to use a potion so that Juliet can fake her death.
The Friar was unable to tell Romeo about the plan so he sent letters; the letters never reached Romeo so he didn't know about the friar plan. The letters never reached Romeo because the people of Mantua thought that the messenger come from a place where the plague reign. When Romeo comes into the room where he sees Juliet's body and thinks that she is dead he kills Count Paris, who is crying over Juliet's body and then commits suicide. After Romeo stabs Count Paris he says, O, I am slain! If thou be merciful, Open the tomb, lay me with Juliet." He then dies. (Act V, Scene III, Line
71-72)
After Romeo had killed himself the Friar arrived at the scene. Then Juliet woke up and the friar told her not to be irrational. Then he heard a noise from outside and runs to the noise forgetting that Juliet just woke up in a drug state. Juliet sees Romeo's dead body and she kills herself with Romeo's axe. The friar should have stayed with Juliet in the tomb so she would have not being able to make any irrational decisions. After the Friar had left the tomb Juliet says a few words before she commits suicide, "Yea, noise? Then I'll be brief. O happy dagger!" Snatches Romeo's dagger. "This is thy sheath; there rust, and let me die." She stabs herself and falls on Romeo's body.
(Act V, Scene III, line 174-175)
Friar Laurence had a good plan to allow Romeo and Juliet to be together and end the family feud between their families but unfortunately he didn't take enough precaution with his plan and ended up causing the tragic events that occurred in the play.