The oath of office. Clearly this did not go 'smoothly.' It is likely that Roberts made 'more' of a mistake than Obama, but Obama 'started it' (when he started reciting the line before Roberts was finished).
Obama starts having a difficult time concentrating. "I can't believe it got messed up," he keeps saying. His chief-of-staff tries to assure him that it's not a big deal and they have important business to attend to. "I can't believe it got messed up," Obama repeats.
Finally, Rahm Emanuel decides they should redo the oath so Obama can stop obsessing about the mistakes and get to work. Obama is receptive and Chief Justice Roberts visits the White House to give it another shot - it goes flawlessly.
Unfortunately, Obama soon slips back into his old thinking patterns: "The redo doesn't count, it should have been right the first time, I'm not really president, why did it get messed up, etc..."
Obama can no longer think of anything else other than the flubbed swearing-in. He decides to resign his office and is working intensively with a certain Doctor Brown in the hopes of going back in time and making sure the swearing-in goes right - the first time.