Technically, these stamps are not forgeries since they never existed but were created and accepted in the philatelic community. Another common stamp forgery is false or enhanced overprinting. An overprint may be very light, or possible non-existent, and the skilled artisan can fill it in so that there is no question of the nature of the overprint. Some of the early overprint forgeries were crudely done using type similar to the original. While others are fairly obvious.
Many collectors save the varieties in watermarks.
In order to producer a variety, skilled forgers split the paper on two stamps, use the front half of the desirable item and the back half of a cheap stamp, glued on upside down. The faked stamp appears thicker than the normal, and may be regummed if it is being offered as a mint stamp. The craze for forgeries has even spread to modern first day postmarks. Unfortunately, even certificates have been forged recently utilizing modern technology. There are experts in almost every nation in the world.