All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
Among one of the Shariah rules is that if a person does something by mistake or out of forgetfulness, then he is not sinful and is not held accountable for it.
Ibn Rajab said:
“Regarding the saying of the Prophet ‘Allah has forgiven my nation for things they forget or do by mistake,’ as to mistake or forgetfulness, the Quran explicitly stated that these are forgiven; Allah says (what means): {Our Lord, do not impose blame upon us if we have forgotten or erred.} [Quran 2:286]; Allah also says (what means): {And there is no blame upon you for that in which you have erred but [only for] what your hearts intended.} [Quran 33:5]
Mistake: to intend to do something by one's action and the result is other than what he had intended, such as: to intend the killing of a (belligerent) non-Muslim, but then he kills a Muslim.
Forgetfulness: to know something but then forgets about it when doing the action.
Both (mistake and forgetfulness) are pardoned; meaning that there is no sin on the person.”
Therefore, if a person mentions a Muslim’s defect or shortcoming in his absence unintentionally, such as if he is very angry to an extent of losing his mind, or if his tongue slips unintentionally, then he is not sinful.
However, if the speaker intended what he said, but then claimed that he did not intend to offend the person he backbit, for example, then this is neither considered a mistake nor an unintentional act, so he is sinful.
As regards the case when the person who was backbit is not harmed by what is said about him, then the extent of the prohibited backbiting, as An-Nawawi said in Al-Athkaar is, “Anything that makes others understand the deficiency of a Muslim is forbidden backbiting.”
Some scholars mentioned definitions that are close to what An-Nawawi said:
For instance, Fat'h Al-Baari by Ibn Hajar reads:
“There is a difference of opinion about the extent of backbiting. Ar-Raaghib said, ‘It is to mention the defect of others without any need to mention it.’ Al-Ghazaali said, ‘Backbiting is to mention your brother with something that he dislikes if he is informed about it.’ Ibn Al-Atheer said in An-Nihaayah, ‘Backbiting is to mention a person in his absence with a defect, even if this defect exists in him.’ Al-Kirmaani said, ‘Backbiting is to speak about a person in his absence with something that he dislikes if he hears about it and it was true about him.’ He also said, ‘The ruling on using a metaphor or making a gesture along with the intention is the same (as making a direct statement).’” [Abridged]
Based on the definition of Ar-Raaghib and Ibn Al-Atheer, it is apparent that mentioning the defect of a Muslim and his fault in his absence is considered forbidden backbiting, even if we assume that the person whom you mentioned does not dislike to be mentioned with that defect and he is not harmed if he hears about it.
Allah knows best.