What is the usual English meaning of the idiom “open Pandora’s box”?

The usual English meaning of “open Pandora’s box” is to release unforeseen troubles.

The idiom comes from the myth in which Pandora opens a vessel and destructive forces escape into the world. Once released, they cannot simply be returned, so the image has become a compact warning about actions with unexpectedly harmful consequences.

In modern English, the expression is usually figurative rather than religious or mythological. Someone may say that a legal case, technology project, political decision, or personal question “opened Pandora’s box” when it created many difficult problems.

The phrase does not normally mean that the person intended harm. Its emphasis is on consequences that multiply or become difficult to control. The expression also preserves the later “box” version of the myth, even though Hesiod described a jar.