For Once Meaning

(idiomatic) For the first time, after many instances to the contrary; in a rare exception to the rule; as an exception to the usual.

Example: 1782, Frances Burney, Cecilia, or Memoirs of an Heiress,
  The invitation for the next evening was accepted, and Cecilia, for once, felt no repugnance to joining the company.
1841, Edgar Allan Poe, Never Bet the Devil Your Head,
  For once I would have taken him up upon his insulting wager.
1865, Charles Dickens, Our Mutual Friend,
  Sell her her waste, please, and give her good measure if you can make up your mind to do the liberal thing for once.
1882 April 27, The Times, The Funeral Of Mr. Darwin,
  In France, in Germany, in Austro-Hungary, in Italy, in the United States, writers of all shades of opinion, for once unanimous, have paid a willing tribute to the worth of our great countryman, ignored in life by the official representatives of the kingdom, but laid in death among his peers in Westminster Abbey by the will of the intelligence of the nation.
1915, Lucy Maud Montgomery Anne of the Island,
  For once in her life she was surprised out of her reserve; she caught her girl in her arms and crushed her and her flowers against her heart, kissing the bright hair and sweet face warmly.
2009 November 9, Jillian Law, Liberal Candidate for Franklin, Now Rudd's on the nose for once, in The Age