Pandora’s Box
Etymology
From Hesiod’s "Works and Days": Pandora, the first woman, was given a sealed jar (Greek "pithos") holding all the world’s evils, and opened it out of curiosity, loosing them upon humanity — only Hope remained inside. The "box" is a mistranslation of the jar by the Renaissance scholar Erasmus. The phrase now marks any irreversible unleashing of trouble.
noun phrase · /pan-DOR-uhz BOKS/
A source of troubles that, once released, cannot be contained again.
What it names
Reach for it when a single well-meaning action unleashes a cascade of consequences no one can undo.
Example
Deregulating the sector opened a Pandora’s box of speculation the government spent a decade trying to close.
Related idea
A Faustian bargain is a deal you knowingly strike for gain; Pandora’s box is trouble released by curiosity or accident, without a bargain at all.