One example of foreshadowing in Act 2.6 occurs in the first two lines of the scene. Friar Laurence says, "So smile the heavens upon this holy act" which means bless this marriage. He then goes on to say, "That after-hours with sorrow chide us not!" This clearly foreshadows the events that occur in the next scene which ultimately lead to the deaths of the newly married couple.
Romeo then goes on to say that "love-devouring death" can "do what he may" once he and Juliet are married because he will be satisfied in this life. This statement foreshadows the tragedy that unfolds as well.
Finally, Friar Laurence compares the couple's passion to gunpowder stating that "these violent delights have violent ends and in the triumph die, like fire and power..." and advises them to practice moderation so the love doesn't burn out quickly. This foreshadows the melodramatic behaviors of both in the tomb at the end of the play which result in tragedy.