Is there anything wrong in connecting the Arduino both on the USB and external power?
I've been doing it for the last couple of days without any issue to power the board and run serial debugging at the same time, usually I connected the DC jack first to a 12V wall wart and once the board was powered up I connected the usb.
Yesterday I did it the other way around, first USB, and when I plugged the 12V jack my macbook shut down hard.
The arduino board now seems dead, no 5V, no 3V3, not sure if the damage went past the power regulation section, will check soon. No magic smoke to be seen though.
The notebook fortunately seems fine, but I'll need a couple of days to check everything.
Any idea what could have possibly gone wrong? My take is that there has been a transient before the Vin went to full 12V where the USB 5V went with inverse polarity through the 1117 regulator and burned it. But I'm not sure what happened next.
I'm getting a new board soon but I'd like to understand what happened and do something to prevent it in the future... a USB hub could help to protect the computer?