You know, it surprises me that people are still talking about the Roberts flub a week later. Sure, it created a situation where the legitimacy of the President of the United States was being openly questioned minutes after his inauguration, but there are certainly more striking examples of oath-related flubbery in American history. For instance:

• In 1841, Chief Justice Roger Taney added the words “I am sofa king we todd ed” to the end of the President’s oath of office, to which President John Tyler replied, “I am—oh, you almost got me.”
• In 1913, Chief Justice Edward White inadvertently administered the oath to himself, temporarily becoming the 28th president of the United States. The matter was cleared up when Woodrow Wilson bludgeoned Justice White to death with the inauguration bible and consumed most of his cerebellum, completing the transfer of power.
• In 1969, President Nixon convinced Chief Justice Burger to perform the oath in a series of farts, in keeping with Nixon’s faith as a complete asshole.
•  In 1993, President Bill Clinton paused during the recitation and emitted a high-pitched tone, which, once analyzed, was found to include the complete oath of office, the Hammurabi Code, and several passages from Eddie Murphy’s 1987 stand-up album “Raw.” Officials at the time decided to “just let it go.”

For a coherent discussion of the requirements of the oath, please consult this Wikipedia page.