A couple years ago I had a brilliant money-making thought. Literally. What if I were to scan a dollar bill, and then print it on my color printer? Would a change machine accept it? Fortunately the thought of spending 20 years in the pokey for attempting such an experiment kept me from following through.
Around the same time I saw some news articles discussing the same problem. Seems that some people had been doing exactly what I outlined above. Though, they must have found some higher throughput way of making money this way. Another thing that kept me from actually trying this was the low earnings rate ... see, the place you'd want to go is a laundromat, and the biggest bill you could foist upon those change machines is $20. At $20 per visit to the laundromat you're not making money at a very high rate. So the risk/reward ratio is low.
In any case at around that time there was a couple related news items. One is that the Feds reached an agreement with the makers of Photoshop and other graphics software to detect images of money, and just show up blackness. Presumably this was meant to foil counterfeiters playing with printers and scanners, but it would also foil legitimate artists making works that include monetary images. Sigh.
The other item was that the slashdot crowd noticed the Feds were also working with printer makers to have the printer makers insert codings in printed output that would let the Feds track the printers.
That is, if a printer were to output some data encoded in the dots being printed ... well ... you could identify the printer used to print the document. This is like the old adage of police detectives trying hundreds of typewriters to identify which one was used to type a kidnapping ransom note. But it's obviously fast-forwarded into the modern age.
Secret Code in Color Printers Lets Government Track You (Electronic Frontier Foundation)
The EFF has researched this and broken the coding. Details are at the page listed above.