Counterfeits?

Depends on where you live. In the United States it is legal. In Canada it is not. Don't know about other places.

In America, possessing a counterfeit banknote is not illegal, so long as there is no "intent to defraud". The relevant federal law is 18 U.S. Code § 472. If, during the course of routine police business, counterfeit notes were to be found in your possession, you could well be arrested for it, but it would be up to the prosecutors to prove that you had intent to defraud. If you are keeping one or two counterfeit notes in a banknote album with a great big "These notes are counterfeit!" sign next to them, I suspect that it might be difficult for them to prove. It would be more problematic for you if you were found with a wallet full of the things, since it could be reasonably argued that the only reason you had a bunch of them in a wallet was that you were planning to go out and (fraudulently) spend them.