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What happens if your friend Alice gives you counterfeit cash, you go to spend it in a store, and they confiscate it?
Counterfeit cash is more easily detectable, and if it's a good counterfeit it will probably fool other people too.
Maybe. I've seen stores uses counterfeit detector pens quite a bit, and in a few cases run the bills through a machine (usually only larger bills). Banks routinely run cash you deposit through fancier machines and probably catch more. I don't do any of these when I get cash from a friend, and I wouldn't necessarily detect whatever it is those tests are doing.
The point is, you can lose money this way, so losing money due to receiving bad Bitcoin is nothing qualitatively new. I'm not saying it is a good thing though.
..... how did we even get into counterfeiting?
"Bad Bitcoin"? Wtf... I know you have some Monero to sell but this is beyond ridiculous