[CrLf]

In the early 20th century there was a high-profile case in Portugal, where a forger managed to convice Waterlow and Sons (a currency printer in the UK, since bought by De La Rue and currently one of the Euro printers) to print batches of portuguese notes.

These were, of course, indistinguishable from real notes. The scheme ultimately failed when a mistake caused the printing of duplicate serial numbers already ordered by the state.

I doubt a scheme like this would work today in the slightest. If in an un-digitized world this didn't last long, in today's world it would quickly be discovered.

https://en.wikipedia.org/wiki/Alves_dos_Reis

[danieltillett]

This is a bit different in that Greece has control over a real printing press. While it would be unlikely to be kept hidden, it should be possible for the Greek printers to avoid overlapping serials numbers.

In practice they could only start their own “unauthorised” printing if it was kept secret since if it leaked the ECB would just issue new notes. This would likely be highly disruptive to the ECB as it was to Portugal.