That led Obama to mix his words up too and the historic swearing-in of the first African-American president briefly became an awkward muddle.
To silence doubts that Obama's presidency might not be legal because he failed to say the oath properly, the two men tried again the next day in the White House and went through the correct, constitutionally mandated avowal, word for word.
Now, Roberts gets his second try - twice: once in a closed ceremony in the White House's Blue Room on Sunday, and then the public one on Capitol Hill the next day.
The White House and the Supreme Court are leaving nothing to chance this time.
After Obama won re-election on Nov. 6, the two men exchanged a copy of an oath card, containing the precise wording, punctuation, and emphasis of the 35-word recitation, an inauguration official said.
According to nearly a century of tradition, when the constitutionally required date of Jan. 20 falls on a Sunday, the president is formally sworn in on that day then holds a public ceremony the next day where he repeats the oath again.
The formal oath that makes Obama the president is the one taken on Sunday, but Monday's ceremony -- and Obama's inaugural speech -- is what will live on in the minds of Americans and TV viewers around the world as the start of his second term.
"Effectively, Sunday is a dress rehearsal for Monday," the presidential inauguration committee official said. "It is another opportunity for them to go through it by the time Monday rolls around."
Judging by the first performance, the two men could use the practice.
On Jan. 20, 2009, Roberts, who was reciting the oath from memory rather than using the customary card, uttered the word "faithfully" out of sequence.
The Constitution requires the president to say, "I do solemnly swear 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 instead prompted Obama to say "that I will execute the Office of President of the United States faithfully." It was a tiny mistake, coming as both Roberts and Obama stumbled at the start of the oath, but enough to raise concerns about whether Obama had been properly sworn in.
The pressure on Roberts this time might not be as intense as in 2009, when a crowd perhaps as big as 1.8 million people watched the ceremony on the National Mall. On Monday, around half that number are expected to attend.
The relationship between Obama and the conservative Roberts, both Harvard Law School graduates, has been prickly in the president's first term.
Their disagreements had the most notable public expression when Obama attacked the court's decision on campaign finance in his 2010 State of the Union address as the justices sat in the audience.
But Roberts gave Obama a victory when he joined the majority of justices in 2012 and upheld the Democrat's landmark healthcare reform, altering the course of Obama's presidency and enabling the president to put his own stamp on history.
However, a wariness lingers.
"I don't think it's a relationship of great trust," said Princeton historian Julian Zelizer. "I do think there's a lot of institutional tension there."
Will Roberts carry the oath card or take any special steps to ensure a smooth recitation?
Supreme Court officials won't say. Spokeswoman Kathy Arberg said the court would not be revealing any of Roberts' preparations.
Still, the inaugural committee official said the Supreme Court and the White House have conferred about the oath.
Roberts' 2009 flub was in contrast to the legendary attention to detail of Chief Justice William Rehnquist, who led the court from 1986 until his death in 2005.
He was known to check out the inaugural platform before the event, stand on his mark and practice the oath. He would memorize the words, and just to be on the safe side, carry a copy of the oath with him.
Variations of the oath have been shared by chief justices and presidents-to-be since the 18th century and the tradition of the chief justice administering the presidential oath traces to 1797, when Oliver Ellsworth swore in John Adams.