Trying to cheat the postal authorities is as old as the Penny Black. A little more than a year after the Penny Black was issued, the GPO realised that some members of the public were re-using Penny Blacks. In candle lit post offices, clerks were finding it difficult to see the black postmark on the black stamp. Consequently the colour of the stamp was changed to red on which it was much easier to see a black postmark.
Later examples arose of forgers making reproductions of current stamps to defraud the GPO. A famous example in London were copies of the 1 shilling stamp which were affixed to telegram forms to record the payment of the amount due to send a telegram from the Post Office at the Stock Exchange. These good quality forgeries, probably used by a dishonest Post Office clerk, went undetected for 26 years until a sharp eyed stamp dealer raised the alarm. Such is their fame that the forgeries often sell for a higher price today than a genuine stamp.
Other forgers have turned their attention to copying rare stamps and covers to deceive collectors. These can vary from amateurish attempts to deceive to highly skilled to defraud philatelists of significant amounts of money. The Royal has a globally respected Experts Committee who issue certificates giving their opinion of items submitted to them.
Finally there are what philatelists call Cinderellas. These are usually described as anything resembling a postage stamp, but not issued for postal purposes by a government postal administration. Cinderellas are a huge and interesting field of collecting. Some cinderellas did pay "postage" for the conveyance of letters and parcels outside the regular postal system, such as by railway and bus companies. Some early cinderella stamps, often crude copies of the original, were produced by stamp dealers keen to help 19th century collectors keen to fill their albums with a space for every stamp that had been issued thus far.