Here are a few neat ones...
During WWII, the U.S., Britain, and Germany all made forgeries of the other side's stamps for propaganda purposes. Here are the more common German forgeries of British stamps.
The first is the German parody version of the 1935 Great Britain 1/2 pence Silver Jubilee stamp.
(Mi 1)
The Germans changed the design to replace King George V with Stalin, and to insert "This war is a / Jewsh war" in place of "Silver Jubilee / Half Penny." (Note the misspelling of Jewish) The dates have also been changed from the original 1910-1935 to 1939-1944, and the Star of David and hammer and sickle appear throughout the design.
The second is the German parody version of the 1937 Great Britain 1 1/2 pence Coronation stamp.
(Mi 2)
The Germans changed the design to replace Queen Elizabeth with Stalin, and to insert "S.S.S.R. Britannia / Teheran 28.11.1943" in place of "Postage Revenue / 12 May 1937". The text "G E R" has also been replaced with "S S S R".
The Star of David and hammer and sickle also appear throughout the design.
The third is the German parody version of the 1937 Great Britain King George VI issues.
(Mi 3-8)

These designs can be identified by the Star of David atop the crown at top center, and a hammer and sickle inserted in the flower at top left and in place of the pence symbol.
These issues were also overprinted in numerous varieties (Mi #9-14, with 32 varieties each), but these are relatively rare.