Pandora's box

A source of unforeseen trouble, as in Revising the tax code is opening a Pandora's box. This equivalent for the modern can of worms comes from the Greek legend in which Pandora, entrusted with a box containing the world's ills, is overcome by curiosity and opens it, thereby releasing them. [Late 1500s]

A receptacle of woes and evils. According to Greek legend, Pandora, the first woman on Earth, was given a jar (it became “box” in the phrase) that she was instructed never to open. Curiosity overcame her, however, and when she lifted the lid, all the evils of the world flew out, not unlike to Eve's eating the forbidden fruit. Someone who does something that leads to widespread disaster is said to have opened a Pandora's box.