In Saki's short story "The Open Window", the character of Vera is a 15 year old girl who seems to enjoy mischief in the form of telling stories that cause fear. As the story reads,
Romance at short notice was her speciality.
This means that Vera's mind is always on the go, especially if it means creating fantasies in her mind.
In the story, Vera tells Frampton (a house guest coming to rehabilitate his nerves) about the reason behind her aunt's leaving the window open on a foggy day. She claims that the reason is that her aunt has a fascination with her husband and her husband's brother, who supposedly disappeared years ago during a hunting trip.
According to Vera, the window is left open in hope of their return. However, this is not the truth: The window is simply open for no reason at all. The men are not missing either, they are actually hunting outside. Vera is making everything up so that, when the men return, Frampton would think it is the ghosts of the men coming back.
The moment in the story where you can almost foreshadow the disclosure at the end is at the beginning, when Vera tells Framptom on account of her aunt's absence from the home: \
MY aunt will be down presently, Mr. Nuttel," ..... in the meantime you must try and put up with me.
Since a foreshadowing is what tells us that something might be happening sometime in the story, the best thing we can get from this specific foreshadowing is that Vera is already determined to cause Frampton some sort of grief. Often, foreshadowing is easier to identifywhen you read the story twice. When you read "The Open Window" twice, the giveaway is that Vera basically warns Framptom from the get go that he will have to put up with her.
When Vera tells Frampton with a shiver that she almost expects the three men to walk in through the window she adds to the expectation that something is about to happen.
Of course, it does happen and Frampton runs away in fear while Vera makes up another story for the reason behind his hasty departure.
Saki uses foreshadowing for the events that occur at the end of the story but leaves us unprepared for unveiling of Vera's character.