Moody's (NYSE:MCO) made a mess of rating subprime debt and other risky instruments. It also said that some bonds help by municipal bond insurers was safe and that these companies should have "Aaa" ratings. Most of that turned out to be wrong and it helped cost investors, banks, and brokerage firms tens of millions of dollars.

Everyone from Moody's customers to Congress wants to know how the ratings could have been so wrong.

Now, the rating company has come up with a novel excuse for another series of mistakes. According to the FT , "Moody's awarded incorrect triple-A ratings to billions of dollars worth of a type of complex debt product due to a bug in its computer models." Several Moody's executives may have known about the mistake some time ago.

Comments from Moody's downplayed the problem. The company said that it adjusted its models from time to time.

The news may get the ratings agency into some real trouble, and it should. If the company was aware of the problem, why wasn't the information passed along to customers who rely on the ratings to make purchases?

Moody's ought to be dragged before regulators and be forced to give an entire accounting of the problem. Perhaps it should pay back customers who made bad decisions because of the errors. Of course, Moody's does not have that kind of cash.

Douglas A. McIntyre is an editor at 247wallst.com.