Different voltage sources across Vin and DC jack So I understand that the USB VBUS circuitry turns off provided there is a high enough vin voltage using the PFET and comparator.

I can see that the DC jack is connected to the Vin pin via a diode. 
However, what is to stop someone from plugging a voltage source across the Vin and at the same time putting across a voltage on the DC jack.

If I incorrectly plugged in a voltage source of a low voltage like a coin cell battery across the VIN pin and also powered the device using the 9&nbsp;V DC jack, wouldn't that then send current through the VIN pin and back to the coin cell battery? 

Is that what would happen in this instance? And if so then shouldn't there be a design feature to prevent users from damaging it like that? Fool/fail safe?
wouldnt that then send current through the Vin pin and back to the coin cell battery? Is that what would happen in this instance?


Yep, that's right.


  shouldn't there be a design feature to prevent users from damaging it like that? Fool/fail safe?


No. VIN can also be VOUT. You can use it to power other things that want a higher voltage.