When you jump a car with another car, or an extra battery, you hook positive to positive, and negative to negative or to a metal part of the dead car. When you connect + to - and + to - you create a short circuit, since the electricity was not meant to travel on that path, it was meant to go through the electrical parts of the car.Further details (and additional impressive photos) in the thread at Reddit.
The insulation on the cables melted from the heat created when the batteries shorted, the battery overheated and caught fire, the electronics may have fried, plastic fuel lines and rubber hoses melted giving more things to burn, and it's all downhill from there.
28 October 2011
The next time you jump start your car...