Pandora's box

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A source of many unforeseen troubles.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun Greek mythology A box given to Pandora by Zeus whose instructions that it must not be opened were ignored with disastrous consequences
• noun A source of unforeseen trouble

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (Greek mythology) a box that Zeus gave to Pandora with instructions that she not open it; she gave in to her curiosity and opened it; all the miseries and evils flew out to afflict mankind