What are some mistakes in Romeo and Juliet that took the tragedy forward?

It could be posited that the first and biggest mistake is love itself. Since the Capulets and Montagues are feuding families, the love between Romeo and Juliet dooms them from the start. Therefore, every action made in order to help foster this love moves Romeo and Juliet closer toward their tragic fate. More specifically, however, there are a few outright "mistakes" which doom Romeo and Juliet. One such mistake is Capulet's misunderstanding of Juliet's grief after Tybalt's death. Since Tybalt is Juliet's cousin, Capulet mistakenly believes Juliet's grief is a reaction to Tybalt's death, and he arranges a marriage between Juliet and Paris to cure his daughter of her grief. In reality, Juliet cares very little about Tybalt. Juliet grieves because of Romeo's exile, and her consequent refusal of the marriage proposal leads her father to threaten to disown her. Under this threat, Juliet seeks out Friar Lawrence for assistance. The Friar then gives Juliet a sleeping potion that eventually leads Romeo to believe Juliet is dead. Romeo then commits suicide, as does Juliet, thus completing the tragedy.