In William Shakespeare's play "Romeo and Juliet," Mercutio's curse is an example of foreshadowing because it hints at the tragic events that will unfold later in the play. After he is fatally wounded, Mercutio curses the houses of Montague and Capulet, saying "A plague o' both your houses!" This curse foreshadows the deaths of Romeo, Juliet, and others as a result of the ongoing feud between the two families. It also suggests that there will be more violence and tragedy to come, as the curse is a symbol of the pervasive conflict that drives the play's action. This moment creates a sense of foreboding and sets the stage for the tragic conclusion of the play.