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 Romeo and Juliet
See what a scourge is laid upon your hate, / That heaven finds means to kill your joys with love.
"Heaven finds means to kill your joys with love": what should unite is what destroys. The result is the opposite of what was expected, since the children's love ends up punishing the parents' hatred.