What is one example of foreshadowing in Macbeth? What is the significance of foreshadowing to the play as a whole?
There are many examples of foreshadowing in Macbeth. The witches' prophecy, the bloody battle in Act I, and the murder of Duncan and its link to sleeplessness.
To answer your question, perhaps one of the most important examples of foreshadowing happens during the murder of Duncan. When Macbeth murders Duncan, he hears:
There's one did laugh in 's sleep, and the other one cried 'Murder!'
This can be looked at as foreshadowing of Macbeth's upcoming problem with sleep. Macbeth even mentions to Lady Macbeth:
Macbeth will sleep no more.
Macbeth knows that what he has done is wrong, that the murder of Duncan will affect him in some way, and that he has already begun to slip mentally. In the end, it is the murder of Duncan that leads to Macbeth's inevitable slip into insanity and insomnia.
The first thane of Cawdor’s betrayal of King Duncan foreshadows Macbeth’s treachery. At the beginning of the play, the Scottish troops are fighting the “Norwayan” (Norwegian) armies. Duncan transfers the thane’s title to Macbeth:
No more that thane of Cawdor shall deceive
Our bosom interest: go pronounce his present death,
And with his former title greet Macbeth.
The thane was so important to Duncan that he was aware of the king’s “bosom interest.” Duncan plans to execute the traitor. Unbeknownst to Duncan, when Macbeth becomes the thane of Cawdor, he will also turn on him. Macbeth murders Duncan, his king, friend, and guest, in his sleep in order to become king. Like the former thane, Macbeth is also eventually killed for treason.
Macbeth is deeply troubled when the witches address him as thane of Cawdor, who still lives and is “A prosperous gentleman.” Soon after this encounter, Ross startles Macbeth by calling him thane of Cawdor, explaining that the present thane is as good as dead: “treasons capital, confess'd and proved, / Have overthrown him.” Macbeth’s realization that one of the witches’ predictions was true sets him on a terrible journey to make their other prophecy come true: that Macbeth would be king.
The duplicity of both thanes of Cawdor makes one wonder if the title carries a kind of curse. It also suggests that treason can happen in the most unexpected situations and in the most apparently trustworthy people.
In addition to the examples of foreshadowing noted above, another important element of foreshadowing occurs in Act 2. Scene 3. just before Duncan is murdered. When Lennox and Macduff arrive at Macbeth's home, Lennox tells Macbeth that they were delayed because the night had been "unruly." The storm made it difficult to travel. Also, Lennox says that "the obscure bird clamored the livelong night," signifying a change in the natural order. The harsh weather and disruption of the general calm in the setting is significant because it relates to the disruption in order for the characters. Macbeth is altering the general scope of fate rather than allowing events to play out in the order in which they should. So Macbeth's manipulation of fate has negative consequences as foreshadowed by the disruption in the setting of the play.