Watch out when removing or installing the battery.
When connecting or disconnecting your battery, you need to be very careful not to drop the spanner or any other object, so it would connect the two terminals and cause an electrical short. Especially if you have a powerful battery, it could result in severe damage caused by the very high current passing from one terminal to the other. The other way in which the accident might occur is when you connect the positive terminal to some other metal element of your car, that is connected to the negative terminal.
That is why you should always remove the negative cable first when disconnecting the battery. And when installing it into the car, connect the positive terminal first. This way the metal parts will be disconnected from the negative terminal while you are working on the positive side of the battery, thus decreasing the risk of an accident greatly. Also, take off any metal jewellery when working on any electrical connections.