Althouse says it was Roberts's fault:
The Chief Justice in fact screwed up the oath. The Constitution requires:
"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."
Roberts left out the word "faithfully." (He also said "President to the United State.") Obama saw the mistake and stopped himself to gave Roberts a chance to fix it. Roberts redid the line, remembering to throw in "faithfully," but putting it in the wrong place after President of the United States and, this time, Obama went along with the wording. Close enough, I guess he figured. I wonder what Barack Obama was thinking. Maybe: Some textualist you turned out to be!
My take here. It reminded me a little of Diana's bungling of her husband's name at that iconic wedding back in 1981. Sometimes, the nerves win.