The oath-taking ceremony of a US President is planned to the last detail but there was a slight blooper when Barack Obama [Images] was being sworn-in as the nation's 44th President.

Coverage: Celebrating Obama

First up, as Chief Justice John Roberts administered the 35-word oath -- watched by some two million people at Washington's National Mall -- Obama started to respond before the Chief Justice had completed the first phrase.

Obama ended up saying the first four words -- 'I, Barack Hussein Obama' -- twice.

Then Obama paused abruptly after Roberts reversed a few words 'that I will execute the office of the president to the United States faithfully'.

The word 'faithfully' had floated upstream in the constitutional text, which actually says 'That I will faithfully execute the office of the president of the United States.'

Obama waited for Roberts to correct the words, realising that something was out of order.

The top judge gave it another try, getting closer but still not quite right: 'faithfully the office of president of the United States.' This time, he omitted the word 'execute'.