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.
