Answers: 

Before you start, always check the type of grounding system the vehicle has. If you remove the positive connector first in a negative ground system, a spark may be created in the process. It could happen if the metal tool you're using to remove the positive terminal connector comes in contact with any piece of metal in the car. If you are working near the battery when this occurs, it might create fire source causing the battery to explode. It's extremely important to remove the ground source first.