The new occurence that Mercutio and Benvolio discuss is that, after the Montagues crashed the Capulet party, Tybalt sent a letter to Romeo's father challenging him to a brawl:
In Act II, Scene iv, lines 7-8, Benvolio announces:
"Tybalt, the kinsman to old Capulet,
Hath sent a letter to his father’s house."
Additionally, Mercutio explains that Tybalt loves a good brawl and is a skilled fighter, which is ironic, since Mercutio later ends up being killed by him:
"More than Prince of Cats. Oh, he’s the courageous captain of compliments. He fights as you sing prick-song, keeps time, distance, and proportion. He rests his minim rests—one, two, and the third in your bosom," (II, iv, 19-22).
All of this is an example of dramatic irony, since Benvolio, Mercutio and the audience now knows that a larger conflict with Tybalt is in the works, yet Romeo does not know it and is feeling on top of the world since meeting Juliet. The reader also knows that Tybalt is a slilled and deadly swordsman, so if Romeo, or any other character go to battle with him, the results will be fatal.