Consider what happens when you attempt to jumpstart a vehicle with a dead, dead, dead battery and, unbeknownst to you at the time, this vehicle is now experiencing a rather sturdy short in its electrical system (which is why the battery is so very dead).
With such a short there is no electrical load on the circuit and so lots of amps start pouring through the jumper cables. Here's what can happen:
Not surprisingly much smoke is generated as well in this particular electrical scenario (which transpired in my garage). The elimination of which is the reason why God invented high volume industrial fans.
All turned out well, though, as far as the vehicles were concerned anyways. The cables...well...it all led to a blog post, so it wasn't a total loss.