What modern idiom means to create many problems by starting one seemingly simple action?

The modern idiom meaning to create many problems through one simple action is “opening Pandora’s box.”

The expression comes from the myth in which Pandora opens a sealed container and releases evils into the world. In contemporary English, it describes an action that may appear harmless but produces numerous unforeseen complications. It is often used in journalism, politics, law, science, and everyday conversation.

The idiom is figurative rather than a literal claim about the ancient story. It can refer to revealing hidden information, beginning a difficult investigation, or changing a situation in ways that cannot easily be reversed. The phrase usually emphasizes consequences that are broad, lasting, or difficult to control.

“Opening the floodgates” is related in meaning but comes from a different image: water being released through gates. “Crossing the Rubicon” likewise describes an irreversible decision but refers to Julius Caesar’s crossing of the river Rubicon, not to Pandora.