Tvindy offers an explanation of "Hoist by One's Own Petard" by giving us a conjugation of the french verb "peter". The phrase as we have it in our book refers to becoming a victim of one's own evil design or to get a taste of one's own medicine. The petard was a device employed in medieval warfare. It was loaded with an explosive device which if successfully fired could cause a degree of damage. if however the device was exploded prematurely, the person lighting it would be tossed into the air by the force of the blast. Now Tvindy's alternate explanation is derived from the verb "peter" which is French for "breaking wind". The petard was given its name by the sound it made when it exploded. An explosive breaking of wind therefore could, as Tvindy surmises, be strong enough to lift one up in the air!? I suspect this could be a challenge for the Mythbusters.
Tvindy also asks about the dervation of "when the Shit hits the fan". The Phrase Finder defines this as an "uproar caused when a previously kept secret situation becomes public". One derivation may be found here. Does anyone know of any other derivation?