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.