Raw gas down the exhaust. Back when I was 16 I had this old Ford truck with a standard tranny and going down the road I would turn the key off with it still in gear so it would make this nice little pop through the exhaust. Everyone thought it was so cool when it popped.
One day it made a huge bang that sounded like someone has just shot off a thirty-aught-six in the cab. It split the header pipe, split the muffler open and totally separated the tail pipe from the exhaust...
After getting my hearing back and fixing that mess I had learned my lesson.
