WASHINGTON - How it happened or who's responsible is a mystery eight years after the fact. But what may have been a simple error — or perhaps something more ominous — has given a multimillion-dollar windfall to a group of oil and gas companies and could cost the government billions of dollars more in the years to come. The Interior Department disclosed Wednesday that a provision was mysteriously deleted from hundreds of federal drilling leases in the late 1990s that would have required producers to pay royalties, once prices reached a certain level, on oil or gas taken from deep...