Arduino getting supply from digital pin?
Yes, this is possible. See this AVR-as-RFID project for an example, which explains:


  The coil actually powers the AVR through two of its I/O pins. Nearly
  every chip out there has clamping diodes on its I/O pins, which
  prevent voltages on that pin from rising above the chip’s supply
  voltage or sinking below ground. These diodes are useful for arresting
  static discharge.When you first hold the RFID tag up to a reader, the
  chip has no power- the supply voltage is zero. When the coil starts to
  pick up power from the RFID reader, these two I/O pins are presented
  with a sine wave, a few volts in amplitude. Anywhere that sine wave
  exceeds the supply voltage, some energy is diverted from the coil to
  the chip’s supply rails, via the clamping diode. The end result is
  that the chip is powered, and the coil’s sine wave is truncated. The
  top and bottom of the sine have been chopped off, and it looks a lot
  more like a square wave now.


So the clamping diodes, intended to protect your AVR from overvoltage, are conducting current from the input pin to the supply rail, powering the rest of the AVR.