When you accidentally get zapped, it makes you jerk away and usually bash your elbow on something else under the hood, which can be very painful, not to mention embarassing. Block the wheels. Work on level ground. Make sure that the Park Brake is set securely. Nothing is quite as embarrassing as being run over by your own car, trust me, I know.
Wheels can have sharp edges and rusty spots.
Sometimes the steel wires from the belts in the tire will come out of the rubber, stabbing you when you run your hand over it. Back injuries are common because of poor lifting techniques, lift with your legs, not your back. Anytime you are working on a car, it is wise to avoid distractions. You need to keep your wits about you. Stay focused on the job at hand, if you find your mind wandering, take a break, let your subconscious work on the problem for a while.