I had an Arduino Nano v3 hooked up to an LCD display and a servo driver from RobotShop. I was powering the whole arrangement using a 12V DC supply that fed into both the Arduino's VIN and the input to the servo driver. This is all on a printed PCB.
Everything has been working just fine, but after a few a days I plugged the Arduino into my PC to modify the code it's running. Both the DC input and the USB input were in place simultaneously. I've done a similar thing before with an Arduino Mego and a 9V supply to no ill effect. This time, however, a capacitor near the Arduino's voltage regulator seems to have burnt out and also a resistor on the LCD display started smoking. The LCD display remains functional but the Arduino does not.
Does anyone know if connecting both USB and an external power supply is known to be problematic? If not, then I may have a separate problem with this circuit.