| a source of extensive but unforeseen troubles or problems: The senate investigation turned out to be a Pandora's box for the administration. |
In classical mythology, a box that Zeus gave to Pandora, the first woman, with strict instructions that she not open it. Pandora's curiosity soon got the better of her, and she opened the box. All the evils and miseries of the world flew out to afflict mankind.
Note: To “open a Pandora's box” is to create an uncontrollable situation that will cause great grief.