Chief Justice Roberts tripped up Barack Obama during the swearing in ceremony. Roberts misplaced the word "faithfully" and the unexpected nature of this surprise no doubt caused Obama to hesitate and stumble.
The Presidential Oath of Office:
"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."
4 months ago