Benjamin Zimmer in his very good blog, Language Log:
Chief Justice John Roberts' administration of the presidential oath to Barack Obama was far from smooth. Early reports differ in saying who stumbled: NBC and ABC say the flub was Roberts', while the AP says it was Obama's. I think both men were a bit nervous, and the error that emerged from their momentary disfluency came down to a problem of adverbial placement.
The Constitution gives the oath as:
“I 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.”
More here.