Try to take a $5 on a color digital copier and make a copy. The page will come out totally full of black toner. The new $5 has a pattern that tells the copier to ‘not copy’.
The back of the bill made us first think we got monopoly money. There’s a big purple 5 on the back.
Link courtesy of Bill Castner — http://en.wikipedia.org/wiki/EURion_constellation