Depending on how you brake the motor different thing can happen. The motor will always generata a back EMF when voltage is abruptly removed, since the magnetic field in the windings will colapse. This spike can create a lot of trouble if not taken into account.
Diodes are use to solve this problem. Also a small resistance large wattage resistor are used as well. A combination of Resistor and cap is also used.
try searching for snubber circuits.
Good Luck
Ivancho