"Not to his father's. I spoke with his man." (2.4.3)
"Where the devil should this Romeo be?" (2.4.1)
"... that Rosaline, Tourments him so that he will sure run mad."(2.4.4-5)
Benvolio and MercutioThe Streets of Verona
"Tybalt, the kinsman to old Capulet, Hath sent a letter to his father's house." (2.4.7-8)
"Romeo will answer it." (2.4.10)
"A challenge, on my life." (2.4.9)
Mercutio and Benvolio are discussing the letter that Tybalt sent to Romeo, wishing to fight, because he attended a capulet party.
"Alas, poor Romeo, he is already dead, stabbed with a white wench's black eye,... And is he a man to encounter Tybalt?" (2.4.14-18)
Romeo deceives Mercutio, because he still believes that Romeo is in love with Rosaline. The ideas that true love is connected to fate and that true love is unexpected is evident here, because we see that Mercutio does not expect Romeo to move on so quickly from Rosaline, but Romeo can't help that he and Juliet are destined to be together.
Shakespeare is indicating, through the use of the motif deception, that love, if not true, can be a lie. One may think that they are in love will someone, but they can be deceived.
Mercutio uses the metaphor "stabbed with a white wench's black eye" to show how intensely he believes Romeo is in love with Rosaline, but really, Romeo has just been telling a lie.