Interpolations may be inserted as an authentic explanatory note, but may also be included for fraudulent purposes. The forged passages and works attributed to the Pseudo-Isidore are an example of the latter. Similarly, the letters of Ignatius of Antioch were interpolated by Apollinarian heretics, three centuries after the originals were written. Charters and legal texts are also subject to forgery of this kind. In the 13th Century, a medieval romance, the Prose Tristan, inserted another prose romance the Vulgate Queste del Saint Graal in its entirety in order to reinterpret the Quest for the Holy Grail through the optics of the Tristan story.
However, most interpolations result from the errors and inaccuracies which tend to arise during handcopying, especially over long periods of time. For example, if a scribe made an error when copying a text and omitted some lines, he would have tended to include the omitted material in the margin. However, margin notes made by readers are present in almost all manuscripts. Therefore a different scribe seeking to produce a copy of the manuscript perhaps many years later could find it very difficult to determine whether a margin note was an omission made by the previous scribe (which should be included in the text), or simply a note made by a reader (which should be ignored or kept in the margin).
Conscientious scribes tended to copy everything which appeared in a manuscript, but in all cases scribes needed to exercise personal judgement. Explanatory notes would tend to find their way into the body of a text as a natural result of this subjective process.
Modern scholars have developed techniques for recognising interpolation, which are often apparent to modern observers, but would have been less so for medieval copyists.