In Shakespeare's Romeo and Juliet, one of the first examples of foreshadowing is when Romeo and his friends are thinking of crashing the Capulets' party. Benvolio suggests that Romeo might find somone new there, better than Rosaline. He says that by comparison, Romeo will see such beauties that Rosaline will seem a crow when compared to these "swans." This is what happens, and this is foreshadowing.
At this same ancient feast of Capulet's
Sups the fair Rosaline whom thou so lov'st;
With all the admired beauties of Verona.
Go thither, and with unattainted eye
Compare her face with some that I shall show,
And I will make thee think thy swan a crow. (I.ii.86-91)
And then Benvolio notes that Romeo has only ever weighed Rosaline against herself, but compared to someone else, she might not fare so well...
Tut! you saw her fair, none else being by,
Herself pois'd with herself in either eye;
But in that crystal scales let there be weigh'd
Your lady's love against some other maid
That I will show you shining at this feast,
And she shall scant show well that now seems best. (98-103)
In Act One, scene four, as the men prepare to leave for the Capulets' party, Romeo expresses a dark feeling of his impending death. This is foreshadowing also.
…for my mind misgives
Some consequence, yet hanging in the stars,
Shall bitterly begin his fearful date
With this night's revels and expire the term
Of a despised life, clos'd in my breast,
By some vile forfeit of untimely death. (113-118)
When Juliet first encounters Romeo, she asks the Nurse who he is as he leaves the party, stating that if he is married, she'll die a virgin, but her description states that her grave will be her wedding bed. This actually is what ultimately happens.
Go ask his name.—If he be married,
My grave is like to be my wedding bed. (I.v.143-144)
In Juliet's very long speech in Act IV, scene three, she worries that she might wake from her drugged sleep before Romeo arrives, surrounding by the bones of the dead. This is more accurate than she could know—and Romeo will be among the bodies. First she wonders if she will not be smothered in such a place:
How if, when I am laid into the tomb,
I wake before the time that Romeo
Come to redeem me? There's a fearful point!
Shall I not then be stifled in the vault,
To whose foul mouth no healthsome air breathes in,
And there die strangled ere my Romeo comes? (32-37)
Then Juliet goes further, wondering if she wakes among the bones and is driven mad by fear—even finding Tybalt's enshrouded corpse—might she not kill herself? In truth, she will kill herself, but not as she imagines: this is more foreshadowing.
O, if I wake, shall I not be distraught,
Environed with all these hideous fears,
And madly play with my forefathers’ joints,
And pluck the mangled Tybalt from his shroud,
And, in this rage, with some great kinsman's bone
As with a club dash out my desp'rate brains? (51-56)
These are all examples of foreshadowing in Shakespeare's Romeo and Juliet.