What phrase describes an action that creates many unforeseen problems, like opening Pandora’s box?

“Open a can of worms” describes an action that creates many unforeseen problems, much like opening Pandora’s box.

The idiom means to create a complicated situation by beginning to deal with an issue. Once the “can” is opened, many separate difficulties appear, making the original problem harder to control. It is commonly used in informal English.

Pandora’s box is a related but distinct expression. In the Greek myth, opening the container releases evils into the human world, while in modern speech “opening Pandora’s box” means starting a process that produces troublesome consequences. Both idioms use an image of containment followed by disorder.

Other English expressions have different meanings. “Break the ice” means to make social interaction easier, “bite the bullet” means to face something difficult, and “hit the nail on the head” means to identify something exactly. Those phrases should not be treated as synonyms for opening a can of worms.