That confuses me more :%, if possible 
I believe you are suggesting a diode reversed, so that it's not normally active and allows current to flow from GND to Vcc (when could that happen?) if necessary. That should prevent reverse voltage (and current) to reach the MCU. Am I correct?
The only situation when I can see this happening is with an inductive load applied on the same power lines of the Arduino and in that case I would use a diode close to each one of such loads, if present. I'm sure I'm missing something, but I just don't see it: apologies.
Thanks a million for your free advices 