This episode discusses five ways you can easily damage an Arduino - so you can avoid them!
Mistake #1: Shorting I/O Pins to Ground
Mistake #2: Shorting I/O Pins to Each Other
Mistake #3: Apply Overvoltage to I/O Pins
Mistake #4: Apply External Vin Power Backwards
Mistake #5: Apply greater than 5V to the 5V Connector Pin
Also learn about an Arduino UNO derivate designed to protect against each potential mishap.