Your servo may behave erratically, and you may find that this only happens when the Arduino is plugged into certain USB ports. This is because the servo draws quite a lot of power, especially as the motor is starting up, and this sudden high demand can be enough to drop the voltage on the Arduino board, so that it resets itself.

If this happens, then you can usually cure it by adding a high value capacitor (470uF or greater) between GND and 5V on the breadboard.

The capacitor acts as a reservoir of electricity for the motor to use, so that when it starts, it takes charge from the capacitor as well as the Arduino supply.

The longer lead of the capacitor is the positive lead and this should be connected to 5V. The negative lead is also often marked with a '-' symbol.