A man named David Rice Atchison was president of the United Sates for one day and didn’t know it. According to a nineteenth-century law, if neither the president nor the vice-president was in office, the president pro tem of the Senate became chief executive. On March 4, 1849, President James Knox Polk’s term had lapsed, and the newly elected Zachary Taylor could not yet be sworn in (it was a Sunday). So for one day Atchison was president. It was not until several months later that Atchison learned of this, as the law was then an obscure one. It has since been changed.