What does Shakespeare foreshadow in Act III?

Act III is the first time Shakespeare presents significant tragedy in Romeo and Juliet. This act is when the death of Mercutio occurs, which is a substantial moment for Romeo and the world of the play. While bleeding, Mercutio spits, "A plague o' both your houses! I am sped" (III.i.90), and this curse is the main foreshadowing that occurs in Act III. This line links back to the initial prologue when the narrator outlines the events of the play and discusses the tragedy that will befall the two lovers. Mercutio reinforces this foreshadowing when he announces the plague.

Mercutio's plague is the first time Romeo hears a foreshadowing of doom, but the audience has already heard this warning. This tension increases the dramatic irony within the play. Shakespeare's use of foreshadowing, coupled with dramatic irony, is one of the reasons Romeo and Juliet continues to be a timeless classic.