It finally happened: I burned an Arduino 
Main story: I connected a 12V DC supply to the Every's Vin and GND pins. The board was brand new and had the default blink sketch preloaded; the D13 LED was blinking as usual. Then, I connected the arduino with a known working USB cable to my computer. At this point I stopped paying attention to the board itself as I never thought there could be a problem. I have connected arduino Due with the exact same power supply and USB simultaneously without trouble.
After a while I tried to upload a sketch and got this upload error and output (Arduino IDE 1.8.12).
avrdude: Version 6.3-20190619
Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
Copyright (c) 2007-2014 Joerg Wunsch
System wide configuration file is "C:\Users\Rick\AppData\Local\Arduino15\packages\arduino\tools\avrdude\6.3.0-arduino17/etc/avrdude.conf"
Using Port : COM10
Using Programmer : jtag2updi
Overriding Baud Rate : 115200
JTAG ICE mkII sign-on message:
Communications protocol version: 1
M_MCU:
boot-loader FW version: 1
firmware version: 6.00
hardware version: 1
S_MCU:
boot-loader FW version: 1
firmware version: 6.00
hardware version: 1
Serial number: 00:00:00:00:00:00
Device ID: JTAGICE mkII
AVR Part : ATmega4809
Chip Erase delay : 0 us
PAGEL : P00
BS2 : P00
RESET disposition : dedicated
RETRY pulse : SCK
serial program mode : yes
parallel program mode : yes
Timeout : 0
StabDelay : 0
CmdexeDelay : 0
SyncLoops : 0
ByteDelay : 0
PollIndex : 0
PollValue : 0x00
Memory Detail :
Block Poll Page Polled
Memory Type Mode Delay Size Indx Paged Size Size #Pages MinW MaxW ReadBack
----------- ---- ----- ----- ---- ------ ------ ---- ------ ----- ----- ---------
signature 0 0 0 0 no 3 0 0 0 0 0x00 0x00
prodsig 0 0 0 0 no 61 61 0 0 0 0x00 0x00
fuses 0 0 0 0 no 9 0 0 0 0 0x00 0x00
fuse0 0 0 0 0 no 1 0 0 0 0 0x00 0x00
fuse1 0 0 0 0 no 1 0 0 0 0 0x00 0x00
fuse2 0 0 0 0 no 1 0 0 0 0 0x00 0x00
fuse4 0 0 0 0 no 1 0 0 0 0 0x00 0x00
fuse5 0 0 0 0 no 1 0 0 0 0 0x00 0x00
fuse6 0 0 0 0 no 1 0 0 0 0 0x00 0x00
fuse7 0 0 0 0 no 1 0 0 0 0 0x00 0x00
fuse8 0 0 0 0 no 1 0 0 0 0 0x00 0x00
lock 0 0 0 0 no 1 0 0 0 0 0x00 0x00
data 0 0 0 0 no 0 0 0 0 0 0x00 0x00
usersig 0 0 0 0 no 64 64 0 0 0 0x00 0x00
flash 0 0 0 0 no 49152 128 0 0 0 0x00 0x00
eeprom 0 0 0 0 no 256 64 0 0 0 0x00 0x00
Programmer Type : JTAGMKII_PDI
Description : JTAGv2 to UPDI bridge
M_MCU hardware version: 1
M_MCU firmware version: 6.00
S_MCU hardware version: 1
S_MCU firmware version: 6.00
Serial number: 00:00:00:00:00:00
Vtarget : 5.0 V
avrdude: jtagmkII_initialize(): Cannot locate "flash" and "boot" memories in description
avrdude: jtagmkII_reset(): timeout/error communicating with programmer (status -1)
avrdude: initialization failed, rc=-1
Double check connections and try again, or use -F to override
this check.
avrdude: jtagmkII_close(): timeout/error communicating with programmer (status -1)
avrdude: jtagmkII_close(): timeout/error communicating with programmer (status -1)
avrdude done. Thank you.
the selected serial port
does not exist or your board is not connected
I turned to look at the board again and the D13 LED wasn't blinking anymore and I can't upload any sketches. Unfortunately I don't know when exactly the LED stopped blinking, and therefore the main chip had stopped responding.
The Every is recognized normally by the computer, the POWER LED is on, and the onboard voltage regulator still provides the 5V and 3.3V pins with the correct voltages so I am assuming it is ok. There are no visibly burnt or destructed components and there was no smoke.
I am assuming the 4809 chip is fried because I can't upload to it. Could that be because I connected both USB + External Power supply? Maybe it was just bricked? And, anyway, is there any way to fix the board?
Irrelevant sidestory: Before even connecting the board to USB I connected D10 of the Nano Every to a darlington pair's base without a resistor. Measuring with a multimeter and an older arduino nano, I found that the base only absorbs 1.4 mA without a resistor when D10 goes HIGH. Therefore, that can't be the reason my Arduino nano every fried. And the preloaded sketch didn't even use D10.
