The first story changing event could have been if Mercutio did not get killed in a fight with Tybalt. Mercutio died after being in a fight with Tybalt, his friends thought he was joking, however he was actually stabbed. “Romeo, Brave Mercutio is dead”(Act 3 Line # 1589).
After Mercutio's death Romeo was very unstable. “This Day’s black fate on more days doth depend. This but begins the woe others must end”(Act 3 Line # 1592-1593). Romeo is saying that he will have vengeance for Mercutio's slaughter, which results in his banishment. If Mercutio were not to get in the fight that killed him Romeo would not get banished for killing Tybalt. Letting Romeo and Juliet live in verona together.