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 Julius Caesar
It is no matter, his name's Cinna
A people that had just applauded liberty attacks an innocent man because of a name: "his name is Cinna." The result is the opposite of what was expected, since the word, handed over to the mob, begins to kill without distinction.
Act III: The Assassination and the Forum, A citizen from the mob (the fourth citizen)
O Julius Caesar, thou art mighty yet! / Thy spirit walks abroad and turns our swords / In our own proper entrails.
Act V: Philippi and the Death of Brutus, Brutus
Subscription
The explanation of each passage, in every device and in every work, comes with a subscription.