When the Treasury Department shelved its plans to sell $5 billion of Citigroup Inc common stock in a public offering, the news came only two days after the bank had said the sale was a go. The delay was a reminder that predicting the future can be a tough exercise.

So imagine how difficult it must be for Citigroup to predict the amount of taxable income it will generate during the next 20 years.

If you think its executives can do that, then you just might believe the $38 billion net value for an item on Citigroup's balance sheet called deferred-tax assets, which represent 27 percent of the company's shareholder equity. Keep in mind, this is the same Citigroup that didn't see the credit crunch coming until it was too late, and wouldn't have survived without a government rescue.