Besides being the most frustrating thing you never want to do, jumpstarting a battery can be downright dangerous. Batteries contain sulphuric acid and when overcharged (via jumpstarting) produce hydrogen gas. If the battery is not properly sealed and incorrect jumpstarting causes a spark, the gas will ignite and the battery will explode and spew acid. Sound unlikely? Every year, in the US alone, more than 6,000 people suffer eye injuries and sometimes blindness because they didn’t get the jumpstarting right the first time.
Morals of the story?
1) Don’t let your battery die in the first place.
2) If it does, read the jumpstarting instructions thoroughly.