One of the most well known example of premonitions of the tragic ending in Romeo and Juliet is when Mercutio says the famous  line: ” A plague upon both your houses”. Mercutio is Romeo’s best friend and cousin, Mercutio is outraged when Romeo tries to pacify his duel with Tybalt, so Mercutio draws his own blade and kills Tybalt and himself in the process. When Mercutio says a curse on both thier houses, i think he means that because of them he died. He puts a curse on their famlies because he blames them for his death. Shakesphere has used this to propheises Romeo and Juliet end.