Romeo's rash intervention between Mercutio's and Tybalt's fight ultimately caused both of their deaths. Romeo might have come forth with the truth, or at least tried to stop the fight without physically putting his friend in danger.
Lord Capulet's insistance that his daughter be married within the week pushes Juliet over the edge. Juliet might have acted a little less obstinate and attempted to assauge and change her father's mind through reason rather than act like a child (remember, Juliet is really a child) throwing a tantrum and challenging her father's dictate.