On root canvas level PG1 and PG2 are created. User1 and user2 have access to PG1 while only user2 has access to PG2. User1 or user 2 creates a processor and corresponding CS in PG1. At this time both user1 and user2 can disable and modify that CS. User2 then copies the processor referencing that CS in PG1. That snippet is then pasted inside of PG2. Since PG2 is not a sub process group of PG1 the CS referenced in that copied snippet by UUID is out of scope for that pasted processor. The CS in PG1 still sees that referencing processor from PG2 and now user1 can no longer disable and modify the CS in PG1.
The pasted processor is clearly out of scope of referenced CS. The processor would still present as invalid when pasted and still reference the CS's UUID in its snippet until property was updated or until processor was moved to a new location that was within scope of the CS. The fix here would be to make sure the CS does not reference any processors that are out of scope. So in this specific scenario it would not block.