Counterfeiting money is clearly a difficult thing for individuals, but surely a country should have no problems counterfeiting another country's currency. Why don't countries do it? For example, for the US to exert pressure on North Korea (or vice versa), it could conceivably print lots of North Korean Won and use that to cause hyperinflation in North Korea. Similarly for Greece to dig itself out of its debt crisis it could just print USD or any other major currency, and convert that into Euros.
My best guess is that this isn't done because it's an act of war, but countries are apparently willing to sling cyberweapons at each other, in which case I don't understand why they don't also do this, at least on a small scale.