Arduino mega fail, suspected power supply issues

My Arduino mega suddenly stopped working. I'm ordering a new one but want to make sure I'm taking care of the underlying issues so I don't destroy the next one.

I was using my Arduino Mega to run 2 TTL to RS485 converters in a loopback test. The converters drew 20ma converters together and I was supplying 12V to Vin and connected to the USB port as well.

From what I have read I don't see why this set up could be an issue.

Maybe I missed something.

Does anybody else have any idea what happened or what I could improve?