Metaphorically speaking, to open a can of worms means to inadvertently create numerous new problems while trying to solve one. Experts disagree on the origin of the phrase, but it is generally believed to be a Canadian or American metaphor coined sometime in the 1950s. Bait stores routinely sold cans of worms and other popular live baits to fishermen, who often discovered how easy it was to open them and how difficult it was to close them. Once the worms discovered an opportunity to escape, it became nearly impossible to keep them contained.
Some experts say the metaphor is a modern extension of Pandora's Box. In the original story, a mortal was warned not to open a box belonging to Pandora. When curiosity got the best of this mortal, she opened the box and inadvertently released numerous plagues on the world. According to legend, the only thing remaining in Pandora's box was a creature called Hope. In this same sense, to open a can of worms means to release a host of often irrevocable problems or complications. As long as the "can" remained sealed, there would be no harm.
It is rarely a good thing to open a can of worms, although the damage control process could prove to be cathartic. An accountant looking for answers to a tax problem could discover evidence of financial wrongdoing by his client, for example. The exposure of that one secret could set off a chain of events with even more dire consequences. Once the accountant decided to open the can, however inadvertently, the worms themselves triggered an entirely new set of problems. By exposing the truth to the light, however, the situation could now be handled honestly.
Sometimes the decision to open a can of worms does not work out so well. History is full of events in which the investigation of one problem has led to the exposure of dozens of other problems lurking beneath the surface. Investigations, such as the Washington Post inquiry into a break-in at the Watergate office complex in 1972, often expose scandals much bigger than the original story. There is often no elegant or efficient way to reseal the ugly truth once someone decides to reveal it.