What are two instances of flashbacks or foreshadowing in "The Pit and the Pendulum"?

In his dark dungeon, the narrator returns to consciousness and recalls how he got to be in this place. He describes

[...] a full memory of the trial, of the judges, of the sable draperies, of the sentence, of the sickness, of the swoon. Then entire forgetfulness of all that followed; of all that a later day and much earnestness of endeavor have enabled me vaguely to recall.

Up until now, he has not even opened his eyes; he has only barely attempted to actually move his body. He has gaps in his memories, or flashbacks, and this eventually helps him to conclude that he is being drugged. These gaps also make him a somewhat unreliable narrator because he simply does not recall some of what has happened to him.

The narrator's moment of "joy—of hope" even while underneath the swinging pendulum seems to foreshadow his eventual escape. He describes "hope—the hope that triumphs on the rack—that whispers to the death-condemned even in the dungeons of the Inquisition." This moment gives us hope, just as the narrator is comforted by it. Even in the worst of moments, of course, we can have hope.

In the first paragraph of "The Pit and the Pendulum," Poe foreshadows the coming fate of the narrator symbolically, when, after looking at the candles and imagining them as angels who might save him, the narrator's mind switches, and has this dark and fearful premonition:

"...the tall candles sank into nothingness; their flames went out utterly; the blackness of darkness supervened; all sensations appeared swallowed up in a mad rushing descent as of the soul into Hades."

In the next scene, the narrator is in a black room, as he imagined. Lying there in "a swoon," a half-dream where he, the narrator, tries to piece whatever he can together, he has the following flashback:

"...These shadows of memory tell, indistinctly, of tall figures that lifted and bore me in silence down—down—still down—till a hideous dizziness oppressed me at the mere idea of the interminableness of the descent."

It is indeed a descent which finds the narrator in a very real hell, where soon the swaying pendulum appears, designed to cut him in two.

What is an example of foreshadowing in "The Pit and the Pendulum"?

In "The Pit and the Pendulum," Poe gives little clues along the way that foreshadow coming events; this helps the reader to predict, in a general way, might happen next.  The first time we meet the narrator, he is groggy and being interrogated; he mentions the Inquisition.  This helps the reader to know that he has been accused of a lack of conversion to the Catholic church, and will most likely be killed (if you know anything about the Inquisition, that is).  However, the first bit of foreshadowing comes when the narrator tells us that he does not receive "the dread sentence of death."  This foreshadows the fact that he is not going to die, but, that something else horrible will happen to him.

Foreshadowing also occurs as the narrator regains consciousness in his prison chamber for the first time.  Fears flash through his mind about torture; he states,

"here came thronging upon my recollection a thousand vague rumors of the horrors of Toledo. Of the dungeons there had been strange things narrated—fables I had always deemed them—...Was I left to perish of starvation in this subterranean world of darkness; or what fate, perhaps even more fearful, awaited me?"

This declaration of his fears is in fact foreshadowing of coming torture.  He mentions that he has heard rumors of the awful torture that had occurred at Toledo, and that he fears that too would be his fate.  Granted, we don't know how he'll be tortured, but torture in general is foreshadowed.

A more obvious instance of foreshadowing occurs when we first learn of the scythe attached to the pendulum.  We understand instinctively that the scythe is going to slowly and painfully slice him to death.  How will he free himself?  Earlier in the story, we learned of rats and how they swarmed his food and the chambers; this perhaps was a foreshadowing of the ingenious plan that the narrator uses to free himself from the pendulum.  Those are just a few examples of foreshadowing in the story; I hope that helped!