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!
If you were a counterfeiter, why wouldn't you just counterfeit some money with an older date? like money with a date from the 70's, that didn't have the same anti-counterfeit stuff that today's money has.
the idea is to give big time counterfeiters trouble. They just can't "move" lots and lots of old bills without raising suspicion, hence the continuous re-design of money.