One break down in communication that we see is of course the fact that Friar Laurence's letter was never delivered to Romeo in Mantua. It was essential for Romeo to have received the letter because it was the only way he could learn about Friar Laurence's deceptive plan to fake Juliet's death. Friar Laurence could have delivered the letter himself or found some other way to get the essential communication to Romeo. Instead, he gave it to Friar John to be delivered on foot. Not wanting to travel alone, Friar John found a fellow friar to travel with him who had been visiting the sick. The authorities suspected that they had both been in an infectious house and quarantined them; therefore, the letter never made it to Mantua, as we see in the lines:
Suspecting that we were both in a house
Where the infectious pestilence did reign,
Seal'd up the doors, and would not let us forth,
So that my speed to Mantua there was stay'd. (V.ii.9-12)
Instead, Romeo's servant, Balthasar, delivered the news of Juliet's death and Romeo believed that it was real. This miscommunication of course led to Romeo's misguided suicide.
A second instance we see of miscommunication is that Romeo does not share his decision to marry Juliet with Benvolio or Mercutio. As a result of Romeo's silence, Mercutio does not understand Romeo's reasons for refusing to challenge Tybalt when he is challenged. Instead, Mercutio believes that Romeo is acting cowardly and refers to his refusal to fight as, "O calm, dishonourable, vile submission!" (III.i.72). Had Mercutio been more aware of Romeo's issues and aware that Tybalt was now Romeo's family member, then Mercutio may have reacted differently. This miscommunication may have saved Mercutio's life, as well as Tybalt's, Romeo's, and Juliet's lives.