Slightly tangential, but related: is a backdated "favor cancel" a fake or a forgery? It is made with a genuine device by a postal employee, so it doesn't seem to be a forgery. And I am not sure it is correct to call it a "fake" either since it was made with a genuine device, though there may be no other or better way to describe it. If there is, I would like to hear it. Intent to deceive or defraud is surely part of the equation. While often intended to create something to deceive or defraud others, a lot of backdated philatelic covers simply stayed in the possession of the person who requested the "favor cancel" at least during their lifetime. In the world of philatelic contrivances, the terminology gets even more confused.
Here's a concrete example, from the world of WWII patriotic covers:


Both of these are backdated, the first by a few weeks, and the second by several years. The second was created with a clear intent to deceive, and thus defraud. Knapp was not seeking to deceive anyone, nor defraud them (she sold her covers for $2.50!), but was simply creating a piece of art in which a backdated cancel was a part. I just find it hard to call either one of these a "fake" or "forgery." I would call the second (a Linto cover) a fraud or hoax, while the first one is simply a favor cancel not unlike modern FDCs in which the first day of issue cancel is usually never cancelled on the first day, or even in the city designated in the postmark.
In the world of philatelic creations (or contrivances, as some prefer to call them) the terminology gets even more fuzzy, as they are fake, but real.