When someone talks about opening Pandora's box, it's not a good thing. Pandora's box is a source of troubles. For example, if you start dating your boss, your friends might say you're opening a Pandora's box.
This expression comes from the story of Pandora, the first woman on earth according to Greek mythology. In the tale, Zeus, the father of the gods, created Pandora as a punishment because his cousin Prometheus gave fire to man against Zeus' orders. While the gods and goddesses gave Pandora positive gifts, like beauty and charm, she was also given qualities that could be used for either good or evil, such as curiosity and persuasion. Pandora was also presented with a jar that Zeus told her not to open. But her curiosity got the best of her and she opened it, whereupon out flew all the troubles of mankind – war, famine and so on. In some versions of the story, Pandora hastily tried to close the jar but the only thing she managed to preserve was "hope."
The tale of Pandora is an origin myth, an attempt to explain the start of something — in this case, why bad things happen in the world. Much like Eve's experience in the Garden of Eden, the world was a perfect place before Pandora opened her jar. Pandora's jar became a box in the 16th century due to a translation error [source: Myths and Legends].