What happens is the opposite of what was expected, or of what someone planned, in a way that reveals something about the characters or the world of the work.
In Othello
I found by fortune and did give my husband;
The handkerchief that Iago used as proof against Desdemona is undone by the one who handed it to him: Emilia, who says she found it "by chance" and gave it "to my husband". Iago's plan collapses by the hand of his own wife, the opposite of what he was arranging.