In Sophocles' Oedipus the King, about two-thirds of the way through the play, a messenger from Corinth arrives and tells Jocasta that Polybus, the man whom Oedipus believed was his father, has died. When Jocasta hears the news, she declares, "Oh, you oracles of the gods, so much for you" (Ian Johnston translation).
When this messenger repeats the same information to Oedipus he has a similar response. He wonders why anyone "pay due reverence to Apollo’s shrine" and claims that "prophetic oracles [are] worthless."
While a number of subtle ironies will exist in this scene, the most obvious irony that Sophocles is leading his audience to realize is that the oracles of the gods do have value and that they are not worthless. The audience alreadys knows that Laius, and not Polybus, was Oedipus' father and that the predictions that Apollo made about Oedipus have already come to pass and are true.