Foreshadowing is a literary technique used by authors to give readers an indication of a future event in the story. Writers use a variety of methods or clues to keep the reader intrigued. It could be in the form of what characters say or do or might even be presented in the setting or the placing of objects. Bad weather, for example, is a standard indicator of some tragic or unfortunate mishap which is to unfold.

Three examples of foreshadowing in The Possibility of Evil are, first, the title itself. The title suggests that the story will expose some malicious incident or incidents, and the reader is, of course, interested to discover what insidious act in the plot inspired the title. It is quite ironic, therefore, to eventually find that the benign and dignified lead character is a malicious character who deliberately sows seeds of ill will in her community.   

A second example of foreshadowing lies in the lead character's name. Adela Strangeworth is an unusual name and creates the expectation that she will display some extraordinarily unusual behavior or perform an exceedingly uncommon task. If one links the name and the title, it predicts that Ms. Strangeworth is not only about to behave bizarrely, but that her actions might also be malignant. She is, at first, presented in such a robustly pleasant manner that the reader is compelled to discover what vindictive act such a wholesome character would want to perform.   

The third example of foreshadowing lies in Ms. Strangeworth's seemingly possessive attitude about the town. Her remarks about the statue of her grandfather and the reference to her roses suggest an unusual desire of being in control of what she might believe is her town, just as the roses are hers, and that she might take things a bit too far. The continuous references to her roses and how precious they are also hint at the fact that something unpleasant might happen to them which is, indeed, what occurs in the end.

There are some other examples of foreshadowing, but I believe that these three are the most pertinent. 

In "The Possibility of Evil," Jackson uses foreshadowing to provide subtle clues about the conflict which will take place. In the opening of the story, for example, Jackson describes Miss Strangeworth which includes an example of foreshadowing:

It bothered Miss Strangeworth to think of people wanting to carry them away, to take them into strange towns and down strange streets….

This line suggests that something bad will happen to her roses and, in fact, foreshadows the story's closing scene when her roses are massacred.

Next, we can find another example of foreshadowing when Miss Strangeworth is walking around the town:

Many people seemed disturbed recently, Miss Strangeworth thought.

This line suggests that there is an uneasy atmosphere in the town and, in doing so, foreshadows Miss Strangeworth's next bout of poisoned pen letters.

Finally, Jackson also uses foreshadowing to hint at Miss Strangeworth's intended victims. The following sentence provides one such example:

Don and Helen Crane were really the two most infatuated young parents she had every known, she thought indulgently.

The use of the word "indulgently" infers that Miss Strangeworth is not being open and honest with the Crane family. While she acts friendly in conversation, she feels, in fact, that they are not good parents and this foreshadows the poison pen letter that she later sends to them.