Invalid device signature when trying to upload a sketch
This can mean one of two things. Either you have the wrong board selected from the Tools | Board menu or you're not using the right version of avr. Arduino uses a slightly modified version of avr to upload sketches to the Arduino board. The standard version queries for the board's device signature in a way that is not understood by the bootloader, resulting in this error. Make sure you're using the version of avr that comes with Arduino. (Donʼt worry about it if your make a fresh install of the Arduino IDE and use a newer board)