If someone else attempts to counterfeit that currencyand pass those COINS, there are sufficient counterfeiting laws on the books to prosecute those counterfeiters.
If it becomes known that the issuer of a particular currency is minting underweight COINS, people will stop accepting that currencyand that company will go out of business.
And on top of blocking bank withdrawals and imposing capital controls Greece would also face the massive problems of introducing a new currency in the form of new COINSand notes and procedures.