Spot-on reproduction of UK-made forgeries of 500zl banknote issued by German occupation authorities. It was professionally manufactured in London in huge amounts and dropped over occupied Poland to help Polish Resistance fund their expenses. 
They were made perfectly, nearly impossible to tell apart from official currency, except for one detail - tiny arrow shafts in one spot on the obverse were not dashed in black as in case of originals. Unfortunate mistake, or omission made on purpose to allow insiders identify the fakes? Nobody knows for sure. 
Germans intercepted one massive drop near Petrikau (Piotrków) and learned to tell them apart. All originals with numbers starting with A12 and A13 were gradually removed from circulation to ease identification of the fakes and administration was trained in spotting the difference. Not many fakes survived to this day, making them fetch very high prices on auctions today. 
 
Check out Money Page of Krause Papierwerke for more interesting period (and not only) banknotes!
 
How else Polish resistance funded itself?
Counterfeiting money in PWPW (State's Printing House) 1940-1942; -18 millions zloty (with respect to 4,000 millions zloty in circulation in GG in 1942 - 0.5%)
Robbery of Bank Emisyjny armored car in Warsaw on August 12, 1943 (action took only 2 minutes):  106 millions zloty (with respect to 5,000 millions zloty in circulation in GG in 1943 - 2%)
The circulation of the "English" counterfeited 500 zloty in only one Inspectorate AK in Piotrkow, August 1944: 15 millions zloty (with respect to 7,500 millions zloty in circulation in GG in 1944 - 0.2%)