Idiom: a complicated (apparently insoluble) problem.

E.g., President Bush's plan to foster cooperative efforts between government and religion-based groups may open a legal can of worms.

"The explosive growth of the Net music business also has opened up a copyright and licensing can of worms." --CNET, April 15, 1998
Metaphor comes from a container of bait, which would have a wriggling, tangled mass of worms, from which it is difficult to extract a single worm required for baiting a fishing pole.