Juliet says these words:
Indeed, I never shall be satisfied
With Romeo, till I behold him--dead--
Is my poor heart for a kinsman vex'd.
Madam, if you could find out but a man
To bear a poison, I would temper it;
That Romeo should, upon receipt thereof,
Soon sleep in quiet. O, how my heart abhors
To hear him named, and cannot come to him.
To wreak the love I bore my cousin
Upon his body that slaughter'd him!
The word "dead" here is set apart by hyphens for a reason. If used with the first part it reads like Juliet won't rest until she sees Romeo taken out. If with the second part, the first part reads as if she won't rest until she can have him, and that her heart is dead for her kinsman (which could mean Romeo or Tybalt depending on how Juliet says it and Lady Capulet receives it).
She talks about having a poison to temper. This word, temper can mean mix or weaken. Lady Capulet takes it one way, Juliet means it to us another way.
Finally, she talks about hating to hear his name and not being able to come to him. Lady takes this to mean that Juliet wants revenge and to get after him, but Juliet means she hates being separated from him. The very last lines show how she wants to "wreak" her love on the body of the man that killed her cousin. Juliet says she wants to express her love, but she leaves it unclear as to how and what love she is expressing.
This double talk creates dramatic irony for the audience because we know a bunch of information that Lady Capulet has no clue about.