Harry Truman's middle name was "S".
Some erroneously assumed that he was named Harry Shippe Truman, after his paternal grandfather, Anderson Shippe Truman. Apparently, the "S" was intended to be ambiguous, so that his maternal grandfather, Solomon Young, could also claim to be his namesake.
When President Franklin Roosevelt died, on April 12, 1945, in Warm Springs, Georgia, Truman, his vice president, was at the Capitol attending a Board of Education meeting. The vice president was summoned to the White House, where Roosevelt's widow Eleanor told him that he had become president.
A spur-of-the-moment inauguration was arranged that evening. Chief Justice Harlan Stone went to the White House to administer the oath to Truman. While reciting the oath, Stone called the new president "Harry Shippe Truman", but Truman responded "I, Harry S Truman, do solemnly swear ..."
I thought I had heard that Truman repeated the oath a couple of days later, to correct the mistake, but I can't find any reference to that on the Web. Perhaps I have that confused with the similar story about Calvin Coolidge, that I mentioned in an earlier post.