Even if you use them as directed, jumper cables still present a serious shock hazard.
Once the positive and negative cables connect to a live battery, any accidental contact between the opposite clamps will produce a spark.
While a spark can definitely harm the person handling the cables, the risk is even greater if there are flammable liquids or gasses nearby.
For instance, if you leave your cables unattended near a puddle of gasoline or oil the results can be deadly.
The best jumper cables will provide generous insulation around the clamps to help discourage accidental electrical discharges. However, negligence can still lead to serious injury.
For the safest jumpstart possible, look for cables that feature heavy rubber or plastic covers around the clamps.