Perhaps it's common knowledge, but it may be worth noting that (at least in the US) a merchant who accepts a counterfeit $100 bill, later notices that it is counterfeit, and does the "right thing" by reporting it simply eats the loss. The Secret Service thanks you, confiscates the bill, and leaves a receipt, but you lose the item you sold and the change you gave.
By contrast, a merchant who accepts a $100 counterfeit bill, does not notice it is counterfeit, and uses it to purchase something else keeps the profit and loses nothing. As a result, it is not in the self-interest of the merchant to look closely at any bills that have already been accepted.
If the goal is to catch counterfeiters, it would seem like there would be a better way to align the self-interest of the person who innocently accepts the counterfeit bill and the goals of the law enforcement agency.
Years ago a friend got a fake $20 from an ATM and reported it to the Secret Service. I don't remember the details, but he told me the Secret Service really hassled him for a while. So it's probably best just to throw the fake bill in the garbage (or burn it to be sure).
or just forget about it being fake, and use it like normal. It's a drop in the ocean, and eating the loss doesn't do you any good, reporting it doesn't do you any good. I know that's selfish, but the way the system has been setup means this is the best thing to do.
True, if you can get away with it, then do that I guess. But there is some risk in passing a fake bill even if you didn't counterfeit it and even if it's not your fault that you came into its possession.
By contrast, a merchant who accepts a $100 counterfeit bill, does not notice it is counterfeit, and uses it to purchase something else keeps the profit and loses nothing. As a result, it is not in the self-interest of the merchant to look closely at any bills that have already been accepted.
If the goal is to catch counterfeiters, it would seem like there would be a better way to align the self-interest of the person who innocently accepts the counterfeit bill and the goals of the law enforcement agency.