I'm not sure exactly what the issue is here ...It seems that with ground at both ends of I0 it is a no-op, I0=0. The real ground end of I0 has to return to a negative voltage to get any current flow from the source.
I don't think your warning will help the OP understand the operation of the circuit, and I don't think it is right anyway.Op amps that approach the ideal do exist in practice, while ideal current sources do not exist in practice. My warning is meant to inform the newbie that you can't build this circuit. Guess I should have said that.
This is wrong. There are many physical processes that can generate a current into a short circuit. The first thing that came to mind was a photodiode, the fourth hit on google gives some data: http://sales.hamamatsu.com/assets/applications/SSD/photodiode_technical_information.pdfThe real ground end of I0 has to return to a negative voltage to get any current flow from the source