Pandora's Box

From Wikipedia:
According to Hesiod, when Prometheus stole fire from heaven, Zeus, the king of the gods, took vengeance by presenting Pandora to Prometheus' brother Epimetheus. Pandora opened a jar left in her care containing sickness, death and many other unspecified evils which were then released into the world. Though she hastened to close the container, only one thing was left behind – usually translated as Hope, though it could also have the pessimistic meaning of "deceptive expectation".