US Presidential Trivia: Which of these President's only took the oath of office once?

The Presidential Oath of Office is famous and can be recited by millions across the world by heart. It reads: I, NAME, do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my ability, preserve, protect and defend the Constitution of the United States.  It is read by each Presidential upon their ascension to office, which officially takes place on January 20 following the November election, or upon the death of their predecessor.  However, like anything else, mishaps can occur.  So, off of Pocket Trivia: US Presidents - Which of these President's only took the oath of office once?  Was it:

• George H.W. Bush
• Barack Obama
• Chester Arthur
• Calvin Coolidge

The correct answer to this one is George H.W. Bush.

In the case of Barack Obama, the President and the Chief Justice stumbled over the oath, which was then retaken at 7:35pm the next day in a private ceremony (incidentally, literally the same time I was proposing to my wife).

Both Arthur and Coolidge took the oath in private, in the middle of the night, following the death of the President before them (Garfield and Harding, respectively).  They then retook the oath in public.  In Coolidge's case, his father actually swore him in the first time.  His father was a public notary, which raised questions about the oath's legality.

Interestingly enough, most scholars contend that the oath is purely symbolic - at 12:00pm on January 20, or upon the death of the current office holder, the person who is next in line becomes President.