Port-Power - How to not power an Arduino (via an I/O port)

I powered up my breadboard circuit, the LED on the Arduino came on - all is well.

Or not, since the Arduino is not connected to power via RAW or ACC. Why then does the Arduino power LED come on?

Turns out GND was connected, and so was 5V to one of the I/O pins. And, there is a path from that pin to ACC: via the internal pullup resistor!