A can of worms, usually paired with the verb open, is an idiom that describes a problem or situation that has the likelihood of being extremely complicated or full of extra problems.
General consensus puts the term origin in the United States at a time when fisherman bought bait in metal cans, likely with a plastic lid since sealing a can with live worms inside would prove difficult. Once the lid was opened to fish, there wasn’t an easy way of putting it back or containing the worms inside. Also the mass of worms would be quite wiggly and look like a real tangled mess, much like a problem or difficult social issue could be.
Pandora’s box is an idiom with much the same meaning and usually paired with the same verb open. The idiom speaks of an action that sets off a series of events that create more problems and complications. The inciting action may seem small or inconspicuous. The word Pandora should be capitalized as it is a name.
This phrase, unlike many idioms, has a definite origin. It comes from a Greek myth about a woman, Pandora, who was given a jar that held all the evil in the world. She was curious and opened the jar anyway.
Both idioms connote actions that cannot be undone. However, a can of worms may simply be annoying or too complicated while Pandora’s box is usually something much worse.
Examples
The Fifa decision to expel Zimbabwe from the 2018 World Cup in Russia has opened a Pandora’s Box on whether Fifa should handle such cases at all. [The Zimbabwean]
And so, apparently, no contemporary politician wants to open the can of worms that would follow any change to the Queen of England’s status as our head of state. [Montreal Gazette]