I used to typeset a number of prominent numismatic journals which were always running stories about counterfeiting in the 19th century. I never ceased to be amazed at the inventive ways counterfeiters would use to get good money in exchange for bad.The problem is, these days they don't barely need to try hard. A deskjet, some heavy stock paper, and away they go.
And the stuff looks REAL! I have invested in a counterfeit detection pen and now use it on all bills, 10 and above, that I get.
