Operation Bernhard: This high grade white British Five Pound note dated 1937, is actually a forgery made during WW2, in German Concentration Camps, with an aim to damage the value of Sterling and for operational use to fund such missions as freeing Mussolini.
The Germans had perfected the paper and deciphered the algorithm for serial numbers, along with engraving almost perfect plates, as much as 3 million pounds were produced.