In Sophocles' Oedipus the King, a messenger arrives from Corinth and tells Oedipus that he has some good news and some bad news. The good news is that the Corinthians want to make him their king; the bad news is that Polybus, the king of Corinth, the man whom Oedipus thinks is his father, has died from "illness" and "from old age."
Because Oedipus had heard a prophecy that he was going to kill his father, this news actually relieves him and he thinks that he "was misguided by my fears."
When Oedipus tells the Corinthian messenger about this prophecy, the messenger says,
My lord, since I came to make you happy,
why don’t I relieve you of this fear? (Ian Johnston translation)
The Corinthian goes on to inform Oedipus that Polybus was not, in fact, his father, but that Polybus had adopted Oedipus as his son after Oedipus had been given, as an infant, by one of Laius' servants to the Corinthian messenger.
Although the Corinthian messenger initially thinks this information will make Oedipus happy, the audience knows as soon as he utters these words that the information the messenger is about to reveal will lead to Oedipus discovering his true identity. The fact is that this information will make him the most wretched of human beings.