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 Inferno
so that three winds were moving from him: from there all Cocytus was freezing. With six eyes he wept, and through three chins the tears dripped and the bloody slaver.
Lucifer beats his wings, and the wind they raise freezes Cocytus, the lake that holds him. The result is ironic: the movement of the wings fabricates the prison.
Cantos XXXI to XXXIV: The Ice and Lucifer, Dante, describing the wings and eyes of Lucifer