There may be one president we're forgetting about.
In the annals of who has been president of the United States, due to interpretation of the laws of succession, there is potentially one man who was president for one day—March 4, 1849. That man was President pro tempore of the Senate on that day, Missouri lawyer David Rice Atchison. His tombstone in Plattsburg's Green Lawn Cemetery tells of his one day as (possibly) POTUS.
The term of President James K. Polk ended at noon on Sunday, March 4, 1849, however the incoming President, Zachary Taylor, refused to be sworn in on the Sabbath. There would also be no swearing in of Millard Fillmore, his vice president. Under the laws of succession at the time, the President pro tempore of the Senate would be considered the Acting Vice President, and therefore president on the Sunday before Taylor could be sworn in.
However, scholars consider it a mere curiosity of history. Atchison was never technically sworn in nor did he exercise any power. Also on that date the Senate was finished for the term. In an interview, he said: "There had been three or four busy nights finishing up the work of the Senate, and I slept most of that Sunday."
Despite the claims of historians, his simple grave marker in Greenlawn Cemetery in Plattsburg reads "President of the United States for One Day, Sunday, March 4, 1849." A statue in front of the Clinton County Courthouse also proclaims the honor on a plaque. He did say afterward, however, that his "administration" was "the honestest administration this country ever had."