Got a new $10 or $20 bill in your pocket? Get it out and look at the yellow numbers. You'll notice that they're actually arranged in a specific pattern at different orientations to prevent counterfeiting:
The EURion constellation is a pattern of symbols found on a number of banknote designs since about 1996. It is added to help software detect the presence of a banknote in a digital image. Such software can then block the user from reproducing banknotes to prevent counterfeiting using colour photocopiers.
Link [wiki] - Thanks collapsibletank!