Medicare is uncharted territory for most of the 10,000 people who come into the program each day. It's not a minefield, exactly, but lurking in the undergrowth are pitfalls and traps that can be costly unless people take care to dodge them. "Avoiding the most common mistakes in Medicare can make the difference between having good financial and health security — or not," says Joe Baker, president of the Medicare Rights Center, a national consumer group. The center hears cons