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 Hamlet
How now? A rat?
Hamlet believes he has killed the king with the thrust at the arras, and strikes Polonius. The gesture that was to fulfill the revenge produces a new victim, the opposite of what was intended.
Act III: The Mousetrap and the death of Polonius, Hamlet
That we would do, / We should do when we would; for this 'would' changes
Act IV: Exile, madness, and the trap, Claudius, to Laertes
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