What happens is the opposite 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 Demons
Our doctors, after the autopsy, completely and insistently rejected the hypothesis of madness.
The town spent the book explaining Stavrogin by illness, and now the doctors "completely and insistently" reject madness. What happens is the opposite of what the town's habit expected, and it takes away its last alibi.
Part Three: The Last Wandering, Stavrogin's note and the chronicler