Greece printing euros would instantly wipe out the value of the debt denominated in euros as well as wipe out most of the value of savings and purchasing power of earnings i .e. Greece would effectively be printing money to inflate its way out of the crisis and thus the Greek inflation rate would soar.
The reason why this is so dangerous is because contagion would be immediate in that German Euro notes would be deemed as being the most valuable and those of the PIIGS nations being marked down in value as each Euro note serial number is marked with the country of origin prefix as the following Italian euro note illustrates that is identified by S –
List of Euro country of origin prefix:
Belgium Z
Germany X
Estonia D
Ireland T
Greece Y
Spain V
France U
Italy S
Cyprus G
Luxembourg 1
Malta F
Netherlands P
Austria N
Portugal M
Slovenia H
Slovakia E
Finla