The average car these days has 20 or more onboard computers. By crossing wires - connecting a positive cable to a negative terminal, or vice versa - you’re going to cause an electrical short that could instantly fry some of those computers. You may not find out the damage until your radio doesn’t work or your power windows don’t work. This could be an expensive repair. Perhaps the damaged microprocessor costs only $100 but it could take a couple hundred more to figure out what is wrong.