What is the standard English idiom for an action that creates many unforeseen problems?

The standard English idiom for an action that creates many unforeseen problems is “opening Pandora’s box.”

The expression comes from the Greek myth in which Pandora opens a sealed vessel and releases evils into the human world. In modern English, it is used metaphorically for starting a process whose harmful or complicated consequences are difficult to control.

The idiom does not usually mean that the first action is physically dangerous. It describes a chain of consequences, often in politics, law, technology, or personal relationships. A decision can therefore be called an opening of Pandora’s box even when no immediate trouble is visible.

The phrase also preserves the later “box” version of the story rather than Hesiod’s original jar. That translation history explains why the familiar idiom differs from the earliest Greek account.