Operation Bernhardt.

During World War II, Nazi Germany used prisoners in the Sachenhausen concentration camp to produce very high quality forgeries of the Bank of England’s £5, £10, £20, and £50 notes. These were considered the most perfect counterfeits ever produced during the war, and was almost impossible to distinguish from the real currency.

The idea was to drop the forged banknotes over England from airplanes and thus wreck the British economy - however, the Germans used these notes to pay their spies abroad instead!

In fact, after the war, a German spy Elyesa Bazna, unsuccessfully tried to sue the German government for outstanding pay!

Operation Bernhard [wiki] | Operation Bernhardt £5 Note