From the longer Wikipedia page 
The Stock Exchange forgery was a fraud perpetrated at the London Stock Exchange during the years 1872 to 1873. It involved forged postage stamps that were applied to telegraph forms and it was only detected over 25 years later.
A fraudulent clerk supplied forged stamps for the forms in order to steal the expensive one shilling fees without depleting the stock of genuine stamps which were subject to audit. The fraud was successful because the forged stamps were only used on certain days and the stamps were convincing forgeries that were not retained by the customers who only handled them briefly. After the telegrams had been sent the forms were filed before later being put in a bag for disposal.
Details of the fraud are impossible to determine with certainty. From investigations carried out by Dr. Ian Ray, it seems likely that a head clerk named George E. Smith colluded with his subordinates T. H. Wright and Benjamin Hind to affix the bogus adhesives to forms. Almost four decades after the known dates when most of stamps were thus sold, Wright and Hind had died, but Smith was still alive, having retired from the post office in 1870's on the grounds of ill health. In 1910, A. J. Waldegrave, formerly Deputy Comptroller and Accountant General to the Post Office, interviewed Smith. Details of that interview have never been made public.
The fraud was not detected at the time and could have remained undetected had all the stamps been destroyed as originally intended; however they were retained by the Stock Exchange for a period, after which they were disposed of as waste paper. Some of the forms eventually came into the hands of stamp dealers, and the fraud finally came to light over 25 years later in 1898 when the young philatelist Charles Nissen noticed several differences from the genuine stamps.
The dates when the fraud was operating are apparent from the dates on the Stock Exchange postmarks applied to the stamps. As a large number of forged stamps were found and 1 shilling was a significant sum at the time it is likely that the fraud was highly profitable for the culprit(s) who were never identified. The forged stamps are now worth more than the originals to collectors.