Updated December 28, 2012
A trivia item known to most hardcore fans is the fact that the term "doombuggy" occurs only in the "breakdown spiel," when riders are told to remain seated in their halted "dooooombuggy," as X pronounces it (that's his voice). Theoretically, you could ride the HM your entire life and never hear the word.
Less well-known is the fact that "doombuggy" was used in press releases given out for the grand opening, so it's been an official term since Day One. It's a pun on "dune buggy," of course, firmly dating the whole business to the late 60's.