The English expression hanky-panky refers to activity that is devious or mischievous. A common form of philatelic hanky-panky is the use of stamps prior to their official release date.
Consider the Souvenir II cover below:
The Souvenir II surcharges were issued on May 27, 1923 -- the date shown on the stamps. But the vertical strips of three on this 48-gram registered cover to Paris are postmarked at Wecker on May 23, 1923 -- four days earlier. Even the poster stamp on the back is tied by a Wecker May 23rd cancel. And there is also a Luxembourg-Ville transit dated May 23rd.
That the return address on the envelope flap has been torn off also suggests hanky-panky. Even today premature use of Luxembourg stamps is unusual, and certainly such use in the 1920s must have been a serious violation of government policy. But knowledgeable collectors know that this cover was not the only hanky-panky that emanated from a certain entrepreneur in Wecker (and Berbourg)!
The cover is overfranked. At 48 grams, the postage would have been 70c plus a 50c registry fee, for a total of 1.20F. The stamps affixed, however, total 1.50F.
French customs was also curious. The customs label indicates that the letter was to be held for customs inspection!
The handed-back cover below shows the stamps actually canceled on the first day of issue: