First time seeing the spark? It's only because the battery is holding a charge and has at least one positive and one negative wire in the connector to the motherboard.

Once you begin seating the connector, the electricity in the battery completes the circuit by moving electrons from the positive wire in the battery, through the machine then back to the batteries negative wire before the plug is fully installed. It can't really be helped, electricity moves a lot faster than we do. The gaps in the connector mid-installation are why you see the arc. You can often see sparks when installing a wall plug for the same reason.

The machine doesn't care because the electricity follows the normal path through the machine. It only arcs when it gets back to it's source before the connection is fully seated.