And it was Roberts who said it wrong on Tuesday. Obama had it memorized, but Roberts, who did not have a script, did not say it correctly, so that's how it became a little mixed up.

From CNN.com

On Tuesday, Roberts, apparently working without a copy of the oath handy on the Capitol steps, started out by reciting a six-word phrase, but Obama broke in halfway through and repeated the first three.

That seemed to throw the chief justice off stride, and he proceeded to mix up the order of the words in the next phrase.

The Constitution sets out the language that should be used in the oath: "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 moved the word "faithfully" back nine spots, and used "to" instead of "of." That threw the president off base, and he smiled and paused to collect his thoughts, then decided to follow Roberts' lead.

But the chief justice at the same time attempted to correct himself.