I have seen jumpstarts that have went wrong cost a couple of grand to fix, what usually happens is people put the leads on two cars, start the car with the good battery in it, start the car with the bad battery in it, then remove the leads without loading the two cars up. when you diconnect the jumpleads is sends out a massive power surge, that in some cases frys lots of expensive stuff...
Even worse if your jumping someone elses car and it ends up being your car that gets fried.
If you must jump start cars best thing to do is get a set of leads with built in surge protecters.