When Martin Sullivan, chief executive of insurance giant American International Group, stood before investors in early December, he delivered remarks that practically oozed assurance.

A.I.G.'s credit portfolio was so sound that the chance of it sustaining economic losses was "close to zero." The company was "confident in our marks and the reasonableness of our valuation methods." And finally, "We cannot predict the future, but we have a high degree of certainty in what we have booked to date." A.I.G.'s stock soared 6 percent.

Reflecting on those comments makes it all the more difficult to stomach today's news. The company disclosed that its portfolio of super-senior credit default swaps had not declined by $1.1 billion during October and November, as Sullivan said it had in December. Instead, it lost about $6 billion in value. Moreover, A.I.G.'s auditors said that the company's internal valuation methods, which its executives exhaustively outlined for investors in order to back up its claims of financial soundness, were flawed. A.I.G. stock plunged 11 percent.

This raises one obvious question: Will Sullivan follow Citigroup's Charles Prince, Merrill Lynch's Stanley O'Neal, Morgan Stanley's Zoe Cruz, and Bear Stearns' Warren Spector to Wall Street's exit ramp?

It's fair to say that it's nothing short of an embarrassment that the chief executive of one of the biggest players in risk management got burned on its securities based on the risk of default.

But it's far from clear that A.I.G. is the next big financial disaster waiting to happen. As Felix Salmon points out on his Market Movers blog, the huge numbers splashed across the headlines today don't tell the whole story. In addition to losing value, the A.I.G. portfolio benefited from so-called spread differential. But because the company can't guarantee that the gain is 100 percent correct, it won't count at all.

Moreover, A.I.G. recognized the increased risk in the mortgage market as early as the end of 2005, when it stopped writing protection on instruments with subprime collateral. What it didn't know then was that so many highly rated products it did protect would turn south so quickly during 2007.

A.I.G. tried to develop its own method of valuing these instruments, and what's alarming is that it still hasn't a clue how to do it. For that matter, no one on Wall Street knows what so many investments out there are worth today, even six months into the full-blown credit crisis.

Sullivan has fought an uphill battle since taking over A.I.G. from Maurice "Hank" Greenberg in 2005, and A.I.G. shares have shed 30 percent of their value since then. While one could make the argument that Sullivan should be ousted, today's $4.8 billion blunder wouldn't be the reason why.

Perhaps other finance chiefs should pay attention to Sullivan's biggest mistake in all of this. The only certainty on Wall Street these days is uncertainty. To convey anything more to your investors will come back to haunt you.