Kills Bugs Dead
The Gravekeeper on May 14, 2009
This might actually work in real life. Sewers are always full of decomposing organic matter (leaves and suchlike in this particular case), which give off highly flammable gases (known as swamp gas, and is one of the culprits behind sightings of "will o' the wisps"). If you were to pump a lot of that gas into a closed space, it would become somewhat concentrated, so a spark from a door lock or a pilot light could cause either an explosion or a flash fire.
I doubt this has happened with sewer gas, but it has happened several times when people leave multiple bug bombs running in their house overnight. Seriously. http://www.snopes.com/humor/follies/bugbomb.asp