Reversing the jumper cables while attempting to jump-start a vehicle can cause a lot of damage, especially to electronic components, including radios, CD players and amplifiers. It is also possible to set the car's battery on fire.
Some cars' electrical systems have safety relays or protective circuitry to protect the powertrain in the event that jumper cables are hooked up backwards. Such a safety relay cuts off the powertrain control module to keep it from being damaged. However, not all cars have this safety feature. Without it, reversing jumper cables can result in destruction of all of the computerized features of the car.