@realjohnboy,
(Article II, Section 1, Clause 8)
http://en.wikipedia.org/wiki/Article_Two_of_the_United_States_Constitution#Clause_8:_Oath_or_Affirmation
Other flubs and redos of historical note:
http://en.wikipedia.org/wiki/Oath_of_office_of_the_President_of_the_United_States
In 1929, Chief Justice William Howard Taft garbled the oath when he swore in President Herbert Hoover using the words "preserve, maintain, and defend" (the constitution), instead of "preserve, protect, and defend". Eventually Taft acknowledged his error, but did not think it was important, and Hoover did not retake the oath.[5][6]
In 1945 President Harry S. Truman's bare initial caused an unusual slip when he first became president and took the oath of office. At a meeting in the Cabinet Room, Chief Justice Harlan Stone began reading the oath by saying "I, Harry Shipp Truman, . . ." Truman responded: "I, Harry S. Truman, . . ."[7]
Besides Obama in 2009, several other presidents have also taken the oath of office a second time for a variety of reasons.
In the cases of Presidents Calvin Coolidge[12] and Chester A. Arthur,[13][14] they both had taken the first oath in a private venue (their residences), in the middle of the night right after being notified of the death of a predecessor (Warren G. Harding and James A. Garfield, respectively), and later retaking it after returning back to Washington. In the case of Coolidge, there was an additional doubt whether an oath administered by a public notary (Coolidge's father) would be valid.[15]
Other presidents (e.g., Woodrow Wilson in 1917) have taken a private oath when Inauguration Day fell on a Sunday, and a second oath in a scheduled public ceremony the next day.[16]