Do you have a laser printer?
Are you a counterfeiter?
Would you place yourself at one of the extremes of the Bristol Stool Scale?
If the answer to any two of those questions is 'yes,' you've got a problem.
Let us assume, for now, that the aforementioned two questions are numbers 1 and 2.
In the late '80s, the U.S. government (along with other governments, quite possibly), started to get a bit antsy about the fact that, with Xerox and several other printer manufacturers churning out color laser printers like never before, it had become remarkably easy for the everyday Joe to counterfeit currency. They voiced these concerns to the printer manufacturers, and the industry came up with a solution: to print stuff on pages without letting users know about it.
Most laser printers manufactured over the last 15 years have a built-in steganographic printing mechanism that prints, onto every sheet of paper that passes through the machine, various information about the printer and the document it has printed. The mechanism writes this information onto the page as a pattern of nearly-invisible yellow dots that become somewhat visible to the naked eye only under blue light.
(image stolen unabashedly from wikimedia; the numbers measure tenths of a millimeter; apparently, paper up-close resembles a relief map of the Rockies)
The Electronic Frontier Foundation, in 2005, determined the encryption scheme for the dots, and even compiled a list of printers they tested. The dots encode the date the document was printed, the time it was printed, and the serial number of the printer. This information still leaves the Feds quite a distance from your doormat, but it was enough for them to bust my counterfeiting ring. I guess I shouldn't go around posting my printer serial numbers and name all over the internet.