Carriers that paid corporate alternative minimum tax for 1998 or 1999 may want to take another look at their tax returns. A change in law that took effect in 1998 exempted businesses with less than $5 million in receipts from the tax. But the Treasury Dept.'s Inspector General for Tax Administration reported in early December that thousands of small businesses had paid the tax for 1998. Neither their tax preparers nor the Internal Revenue Service had caught the error. The overpayment averaged nearly $12,000 each for more than 2000 businesses.