Via The New Yorker:

....one of the critical clues about Enron’s condition lay in the fact that it paid no income tax in four of its last five years. Enron’s use of mark-to-market accounting and S.P.E.s was an accounting game that made the company look as though it were earning far more money than it was. But the I.R.S. doesn’t accept mark-to-market accounting; you pay tax on income when you actually receive that income. And, from the I.R.S.’s perspective, all of Enron’s fantastically complex maneuvering around its S.P.E.s was, as Fleischer puts it, “a non-event”: until the partnership actually sells the asset—and makes either a profit or a loss—an S.P.E. is just an accounting fiction. Enron wasn’t paying any taxes because, in the eyes of the I.R.S., Enron wasn’t making any money.

If you looked at Enron from the perspective of the tax code, that is, you would have seen a very different picture of the company than if you had looked through the more traditional lens of the accounting profession. But in order to do that you would have to be trained in the tax code and be familiar with its particular conventions and intricacies, and know what questions to ask. “The fact of the gap between [Enron’s] accounting income and taxable income was easily observed,” Fleischer notes, but not the source of the gap. “The tax code requires special training.”