You wouldn't tackle a wiring project in your house without turning off the power at the source and then double-checking at the site of the problem, right? (Well, let's hope you wouldn't, anyway.) Same thing with your car. A little laziness might get you zapped.
Some automotive pros say that the current in your car isn't strong enough to really hurt you but it's always better to be on the safe side, which means assuming that you could sustain injury [source: AA1Car]. Even if you don't suffer an electrical shock, you could easily start an electrical fire, or possibly overload your car's wiring or other electrical components. So, you should know what you're dealing with ahead of time. Make sure your car is off when it needs to be off (which would be most of the time) and remember to disconnect (and then isolate) the battery's negative cable. And if you drive a hybrid car with a high-voltage battery, avoid touching the battery at all costs. Since hybrids are designed specifically for heavy-duty electrical power, the battery has much more shock potential than an average car and can cause injury on contact.