I can draw and deface a bank note, does that make fiat not "perfectly fungible"?
imagine this:
- you payed in a store with that dollar
- the store get robbed and the owner got killed
- one employee remebered this dollar was still there
- news reports this with an image with that dollar
is this dollar still fungible or will the thief has problems paying with it?
Assuming the merchant that may accept it is paying attention to the bills coming in...yes they would have problems paying with it.
Most people aren't paying attention to the serial numbers on their bills.
It's not accessible for many to lookup which serial numbers of bills were tied to a specific crime/theft.
luckily in this example mallard made it easy by drawing an image on the dollar and made it unique and easy to remember