Even though I like to work on cars, I have an extremely healthy respect for how dangerous they can be. Jump-starting cars has always been something I'm particularly careful with. This site give you some very graphic photos of why I'm so paranoid (note: no injuries, except to lead cell batteries).

Nowadays I'm just as worried about zorching the electronics of a modern car. If you screw up and hook something wrong up on my Alfa, all you'll get is some burned wire. Do it wrong on today's cars, and you could fry the computer system ($$$).