Many lies and secrets can be found all throughout Romeo and Juliet, especially with respect to the characters Juliet and Friar Laurence.
Juliet kept a secret from her parents when she married Romeo. Both she and Romeo conducted their entire romance in secret. Romeo had to secretly enter the garden after the ball to see her because, if he had been caught, he would have been killed. Juliet also secretly sent her nurse out into town to meet Romeo and inquire what his intentions were for marriage. Romeo even tells nurse to have Juliet "devise some means to come to shrift" that day, meaning to pretend to go to confession in Friar Laurence's chamber where Friar Laurence will marry them in secret (II.iv.166).
We can especially see an example of a lie through Friar Laurence's later devised plan to fake Juliet's death. Since Juliet is not truly dead, faking her death counts as a lie. When Juliet's father decides he needs to distract Juliet from what he perceives to be her grief over Tybalt's death by having her marry Paris, threatening to disown her if she refuses, it becomes very necessary for Juliet to take desperate action. She obviously cannot marry Paris as that would make her guilty of the sin of polygamy. She goes to Friar Laurence for help and threatens to commit the sin of suicide if he does not think of a way to help her. He tells her to let no one know she is faking her death, not even her nurse. He further says that the potion will make her look like she is dead for 42 hours, as we see in his lines:
Each part, depriv'd of supple government,
Shall, stiff and stark and cold, appear like death;
And in this borrowed likeness of shrunk death
Thou shalt continue two-and-forty hours. (IV.i.103-06)
Since the potion makes everyone believe she is really dead, having Juliet fake her death is an excellent example of a lie.