Hospitals in nearly half the states in the nation now say they won't bill patients for medical mistakes, including operating on the wrong body part or the wrong person, or giving someone the wrong blood. The list has more than doubled since February, when an analysis showed that hospital associations in 11 states urged their members to waive payment for "never events." It's not clear how many private patients or their insurers are still billed for medical mistakes, but a recent study by the federal Agency for Healthcare Research and Quality estimated that preventable errors that occur during or after surgery may cost employers nearly $1.5 billion a year.