An interpolation, in relation to literature and especially ancient manuscripts, is an entry or passage in a text that was not written by the original author.
Interpolations originally may be inserted as an authentic explanatory note (for example, [sic]), but may also be included for fraudulent purposes.
Similarly, the letters of Ignatius of Antioch were interpolated by Apollinarian heretics, three centuries after the originals were written.
[note 1] However, most interpolations result from the errors and inaccuracies which tend to arise during hand-copying, especially over long periods of time.
In a majority of cases, in response to late theological developments, the editors would have interpolated "clarifications" into the original text.