BUCKEYE LAKE, Ohio — At least the Ohio Department of Natural Resources can admit its mistakes. And it has made big ones with Buckeye Lake, the largest being its decision decades ago to sell the backside of the dam for private development. Now, 370 homes sit atop the 4.1-mile, 177-year-old earthen dam, not to mention nearly as many patios and docks and trees that, over the years, have converted a structure intended to hold back millions of gallons of water into a lakeside home and garden show — to the possible peril of thousands who live and work downstream.