Mencius - Book 2, Part 2, Chapter 9B

Ch'an Chiâ accordingly saw Mencius, and asked him, saying, 'What kind of man was the duke of Châu?' 'An ancient sage,' was the reply. 'Is it the fact, that he appointed Kwan-shû to oversee the heir of Yin, and that Kwan-shû with the State of Yin rebelled?' 'It is.' 'Did the duke of Châu. know that he would rebel, and purposely appoint him to that office?' Mencius said, 'He did not know.' 'Then, though a sage, he still fell into error?' 'The duke of Châu,' answered Mencius, 'was the younger brother. Kwan-shû was his elder brother. Was not the error of Châu-kung in accordance with what is right?

'Moreover, when the superior men of old had errors, they reformed them. The superior men of the present time, when they have errors, persist in them. The errors of the superior men of old were like eclipses of the sun and moon. All the people witnessed them, and when they had reformed them, all the people looked up to them with their former admiration. But do the superior men of the present day only persist in their errors? They go on to apologize for them likewise.'
~ James Legge translation via nothingistic.org ~

Go here to read the introductory post to this serialized version of the Works of Mencius.