Idiom: a complicated (apparently insoluble) problem.

Can also imply that solving it will be messy or unpleasant (depending on how you react to tactile sensation of inserting your fingers into a can of worms)

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.

One of the infamous items from the first two Earthworm Jim games. The can of worms gives you a continue.

What makes it a bit of a legend is how it was handled in the games. In the first game, the manual stated:

"Finding the hidden can of worms is one way to get a continue. The other? Well, that's fuel for thought..."

The primary method of getting continues was collecting 60 fuel cells in Andy Asteroids?, but the can of worms? Well, the manual for Earthworm Jim 2 clearly stated:

"Can of Worms: Okay, we lied to you in the first one. There was no can of worms."