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!
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.
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.