Stock Exchange Error Reviewed: The Midwest Stock Exchange in Chicago said it is reviewing procedures that caused a subsidiary to get millions of dollars in interest income to which it was not entitled. It said some payments were made to Midwest Clearing Corp. before securities bought by brokerage firms were delivered to the purchasers and the clearing firm earned $1.8 million in interest on the payments just in the last 12 months. "It's clear we were getting . . . money we weren't entitled to, so someone was getting disadvantaged," exchange spokesman Roger Hendrick said. He said the procedures were discontinued and federal regulators had been notified.