What are some dramatic ironies in Romeo and Juliet?

When Romeo and Juliet first take notice of one another and begin to speak in Act 1, Scene 5, the audience already knows that each of them is the child of their father's enemy.  Though they do not know each other's names yet, they fall in love at first sight.  However, the audience knows -- before they do -- that their love is going to be terribly problematic even before they know themselves.  

In...

When Romeo and Juliet first take notice of one another and begin to speak in Act 1, Scene 5, the audience already knows that each of them is the child of their father's enemy.  Though they do not know each other's names yet, they fall in love at first sight.  However, the audience knows -- before they do -- that their love is going to be terribly problematic even before they know themselves.  

In another example, we know that Romeo has been exiled from Verona for the murder of Tybalt, Juliet's cousin, before either Romeo or Juliet finds out.  This builds tension while we wait for them to learn that there are now even more insurmountable stumbling blocks placed in their path: the increased and renewed animosity between the families as well as Romeo's removal from the city.

However, the most prodigious example of dramatic irony in the play is when Romeo does not get the letter that explains the plan for Juliet to drink the potion that will make her appear as one dead.  Instead, he hears that she is actually dead (which is the story given out to everyone but Romeo via this mislaid letter), and so goes to buy a terrible poison that will "dispatch" him, too.  We know that Juliet is not dead, but Romeo does not.  He goes to her tomb, slays Paris, drinks his poison, and dies.  Meanwhile Friar Lawrence arrives, saying, "Fear comes upon me. / O, much I fear some ill unthrifty thing" (5.3.139-140).  We ought to feel the same, knowing that Juliet is about to awaken and find Romeo dead by her side.  This is, by far, the most upsetting example of dramatic irony in the play.  There have been other delays and misunderstandings, etc., but there is no coming back from this one.  Romeo's ignorance of Juliet's plan leads him down a path that ends in the ultimate tragedy.  Finding Romeo dead, Juliet kills herself, and thus the play ends.