It's possible to cause a power surge or spike in voltage causing damage to your on board computer when disconnecting the leads. Once the car with a dead battery is jumped its recommended to turn the headlights on to absorb any spikes in power. I read there are jumper cables now with a "computer protector" on the leads to prevent power surge.
I've instructed my wife some time ago, DO NOT under any circumstances jump someone's car if they have a dead battery, I don't want to risk any damage from people not knowing any better.
CHOICE - ...jump start a car
