Both the Chief Justice and the President Flubbed the Oath:
Little Green Footballs says that "it was Justice Roberts who messed it up, not Obama." But it appears to me that the President had the presence of mind to pause after catching the Chief Justices's error and then repeated the error himself. First, here is what the Constitution says:
"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."
Here is the transcript (which does not indicate the President pausing to enable the Chief Justice to correct himself):
ROBERTS: Are you prepared to take the oath, Senator?
OBAMA: I am.
ROBERTS: I, Barack Hussein Obama...
OBAMA: I, Barack...
ROBERTS: ... do solemnly swear...
OBAMA: I, Barack Hussein Obama, do solemnly swear...
ROBERTS: ... that I will execute the office of president to the United States faithfully...
OBAMA: ... that I will execute...
ROBERTS: ... faithfully the office of president of the United States...
OBAMA: ... the office of president of the United States faithfully...
ROBERTS: ... and will to the best of my ability...
OBAMA: ... and will to the best of my ability...
ROBERTS: ... preserve, protect and defend the Constitution of the United States.
OBAMA: ... preserve, protect and defend the Constitution of the United States.
ROBERTS: So help you God?
OBAMA: So help me God.
ROBERTS: Congratulations, Mr. President.
Here is the video so you can judge for yourself: