During WWII "almost perfect" forgeries of Bank of England Pound notes were produced by prisoners in a German concentration camp. The project was code named "Operation Bernhard". The German intent was to ruin the British economy by flooding them with hundreds of millions of counterfeit pounds.

There are a number of very small but discernible differences between the Operation Bernhard counterfeits and genuine notes. A counterfeit has a dull look in Britannia eyes, her lower lip not as full, less clarity on the cross at top of her crown, fleur-de-lis on the crown not as well defined, and the marking on the vase and the shadow around it not as dark. See side-by-side comparison
of enlarged Britannia. Genuine one is on the left. Can you find other differences?