Hooking up jumper cables in the backwards has the potential to cause an explosion. The reaction occurs if hydrogen gas builds up around the battery and a spark ignites the gas.
The correct way to apply jumper cables is to attach the red end to the positive terminal of the dead battery first, then the other red end is attached to the positive terminal of the charger battery. The black end of the cable is then attached to the negative terminal of the charger battery, and the other black end is attached to the negative terminal of the dead battery. The cables are removed in the reverse order of their attachment.