Dear Wordminstrel: "Hoist on one's own petrard" reminds me of an episode at the swimming hole, a small cow pond, when I was perhaps six. We were all skinny dipping. One junior petomane
was blowing bubbles from both ends. A couple older boys grabbed him, and held a match to the terninus of his gasline.
A blue flame shot out of and into the orifice. If it hadn't been for the two guys holding him, he would have gone into orbit, hoist on his own petard.
Here's a URL, not for the fastidious:
http://www.ooze.com/ooze13/petomane.html