Many advisers believe that the interest-charge Domestic International Sales Corporation (D.I.S.C.) is the last great international tax planning idea because a portion of the export profits are taxed at low rates for shareholders that are individuals. Some believe it is even better when the benefit is channeled to a Roth I.R.A. that provides tax forgiveness rather than tax deferral. But in a recent case won by the I.R.S., we see how too much of a good thing may be bad.