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 Purgatory
and if you cannot remember, she answered, smiling, remember now how today you drank from Lethe; and if from smoke one infers fire, this forgetfulness clearly concludes guilt in your will, which attended to the other part.
Lethe, which exists to erase guilt, becomes the proof that guilt existed. What should absolve incriminates, and the result contradicts the expectation of whoever hoped the river would bring relief.
Cantos XXX to XXXIII: Beatrice, Beatrice, to Dante, using Lethe as proof