I have an Arduino UNO R3. I was working on this excercise and I could not get the servo working. I thought the USB port may not provide enough power, so I plugged a 9V/1A adapter into the barrel jack as well. I also pressed the reset button. I noticed the edge of the PCB between the USB port and the barrel jack getting hot, and I unplugged the Arduino.
When I plug the USB cable in now, the "L" and "ON" LEDs light up, but there is no activity on the RX/TX LEDs and my computer does not recognize the Arduino (no /dev/ttyACM0 shows up). Looking at the edge of the PCB, I see a bit of a white powdery residue where it was heating up, although I am not sure if that was there before.
I'm worried that I may have burned something out in the PCB. I may have had the 3 pin header for the servo misaligned. Can anyone offer me some insight into what may have happened?
EDIT: Here is the edge of the board:
Sorry, I've already scraped off a bit of the powdery stuff, so it is harder to see now.