I don't know how it happens in the US, but in France, during several years, I had an error-code SSN. My papers had been misfiled at birth, due to the unreadable writing of the cleric who recorded my birth, and it took 2-3 years to fix this.
This error-code SSN was full of 0s, so obviously shared with others.
I wouldn't be surprised if some migrants waiting for confirmation of paperwork also had some shared temporary special-code SSN.
