Forgeries
Forgers have not only manufactured stamps for the philatelic market, they have added forged cancellations to those stamps. This was especially common in the late 19th century and early 20th century, when huge numbers of inexpensive stamps were forged for the packet trade.
Forged cancellations have also be applied to genuine stamps, in cases where the stamps are worth much more postally used. In addition, where rare cancellations are desired by collectors, those cancellations have also been forged.
Cancellations also may be used to prove that certain philatelic items are genuine. For example, forgers have fabricated many supposedly valuable postal covers by adding genuine stamps and forged postal markings to pre-stamp covers. A cover can be shown to be genuine if a genuine cancellation "ties" the stamp or stamps to the cover; that is, if a genuine cancellation runs continuously over the stamp and adjacent portion of the envelope, although one still may need to rule out the possibility that the cancellation was added later. Similarly, stamps that were cut in parts and used for a portion of the full value as splits can only be shown to have been so used if a genuine cancel ties the stamp to the cover or piece of cover.
Read more about this topic: Cancellation (mail)