Japan - forgery
There are two main types of forgeries. Stamps printed to fool the postal authorities and stamps printed to fool the collectors. Even the first stamp of the world, the celebrated Penny Black, was forged the very first year of issue (1840). During these early years, in fact, most forgeries were printed to avoid paying the postal rates.
Cape of Good Hope - forgery
Just a few decades later, however, several shady individuals had realized that there was money to be made in the stamp business. By the turn of the century the "great" counterfeiters were at "work". Some of the most famous were Jean de Sperati, Francois Fournier, the Spiro brothers, Georg Zechmeyer and the so called Boston League led by Samuel Allan Taylor. Forgeries, correctly identified can, as well as genuine stamps, fetch quite high prices once they reach the market today. Unfortunately with the help of modern technology the forgers seem to have reached another heyday and homemade forgeries are openly for sale at the big Internet auctions.
Brunswick - forgery
Very few of the rare and valuable stamps have escaped the attention of the forgers. Among their favourites are the pre-Italian and pre-German states (an example above), the early cantonal stamps of Switzerland and most British colonies. Primitive stamps are of course not so difficult to counterfeit, and that's why Indian States or the type of stamp shown below, have also been extensively targeted.
Tibet - forgery
The collector soon learns to correctly identify the most obvious forgeries. If you are considering the purchase of a more expensive stamp however, you can spare yourself much disappointment by assuring yourself that a certificate of authenticity by some well known authority accompanies the stamp.