Vin strangeness on Mega 2560

I had a mega smoke today.

Looking at the schematic, VIN goes directly into the 5v regulator. However if I connect USB power, I can read 5v at VIN. I can't see how in the schematic this should be.
It also resulted in the smoking a mega, and I'm hesitant to connect another until I understand how this is happening.
As far as I can see, no voltage should appear at VIN if using USB for power.

Hopefully someone can shed light on this.

It is easy. Damaged 5V regulator can cause this.
Important thing is to avoid of the same mistake. Normally, Arduino should completely work from 5V USB.
Be careful with connecting the broken Arduino to the USB. It could be much expensive...

If you let the smoke out of your Mega, it’s time to throw the Meg away.

Regulators generally have an internal diode from input to output, in any case…