Step 1: Apply 12V to the 5V input.
Step 2: Your Arduino Nano is now destroyed.
I forgot to disconnect a jumper on my breadboard before popping in my handy breadboard regulator. Fortunately I didn’t have any other parts on the breadboard. It looks as though the USB controller survived so I may be able to just replace the Atmega328 and get it going again.
So, if you’re thinking about connecting 12V to your Arduino’s 5V input, don’t do that.