The Vampire Diaries: Nightfall
L. J. Smith
In the beginning of this book, Damon, one of the main characters, is spying on a girl in her room. (Damon is a vampire.) The girl's name is Caroline and she is also one of the main characters. Caroline is behaving very oddly. She is standing in front of her mirror, talking to herself. But when Damon looks closer, he can see that the mirror Caroline's mouth isn't moving when the real one speaks. And when he looks at the image's eyes, they turn red for just a second. This is the 5th book in this series, and all sorts of supernatural things have happened so far. So I knew right away that this was a bad guy in Caroline's mirror. But Damon didn't. He probably could have realized that something bad was going to happen later with the person in the mirror, but chose to ignore it.
Later in the book, Caroline is with some other characters in the book, asking if she can go with them to see a friend that was recently brought back from the dead (weird I know). This person that was brought back to life is Elena, and before she died, Caroline hated her. So the other characters were suspicious, but let her come anyways. As it turns out, during the middle of their "meeting", Caroline gets angry at Elena and becomes possessed by something. She starts saying dark spells and is practically thrown out of the house by the other characters. This foreshadowing was for both the reader and the other characters. They realize that there is something seriously wrong with Caroline, and that they're in for another supernatural ride.