What happens is the reverse of what was expected, or of what someone planned, in a way that reveals something about the characters or about the world of the work.
In the Odyssey
you are not Odysseus, my father, but some spirit enchants me, so that I weep and groan all the more.
Telemachus has spent the poem longing for his father, and when his father stands before him, he denies that it is he. The expected reunion produces the reverse, refusal, and shows how far the pain of waiting had hardened the boy.