Romeo and Juliet
Friar Laurence counsels, and fails to calm down, both Romeo and Juliet at different points in the play. He clearly cares about both of the lovers, but his role is complicated: his choices lead to Romeo and Juliet's deaths. No matter how many times he warns Romeo not to be too hasty, Friar Laurence fails to take into account just how hastily Romeo might respond to the news of Juliet's death.
And this isn't his only big mistake. Even more disturbing, Friar Laurence runs out on Juliet at the end of the play when she refuses to leave Romeo's side. In his haste not to get caught with the mess he's just made, he abandons Juliet at the moment when she needs him most.
The Nurse is Juliet's mentor, and she's one of the people who help facilitate Juliet's marriage to Romeo/ sexual maturity. But as Juliet grows into a woman over the course of the play, the Nurse's guidance becomes more and more inadequate. By the end of the play, she betrays Juliet completely when she tells her to forget Romeo and marry Paris. Juliet rejects her as a counselor and is left painfully alone. By failing poor Juliet, the Nurse helps contribute to the play's final tragedy.