I was first struck by the expression "parade of horribles" back in April 2008, when then-Senator Barack Obama used it to describe testimony by General David Petraeus and Ambassador Ryan Crocker about what might happen if U.S. forces withdrew from Iraq too hastily. I wrote a Language Log post about it, tying it to another expression that was in the news at the time: "false terribles," used by Rob Lowe to describe things that his nanny accused him of doing. "False terribles" turned out to be pretty much a one-off, but "horribles," usually of the parading variety, have shown up again and again in legal discussions, most recently in the Supreme Court's health care decision on Thursday — which featured, in Justice Ginsberg's pungent opinion, a "broccoli horrible" (referring to the slippery-slope argument that if government can make you buy health insurance, they might someday make you buy broccoli, too).
For a full explanation of how the legal putdown took shape, read my latest Boston Globe column (online now, in print on Sunday). I trace how "the parade of horribles" emerged as a satirical Independence Day tradition in mid-19th century New England, then made the metaphorical jump into discussions of judicial argumentation c. 1921, thanks to the legal scholar Thomas Reed Powell. Since then, the expression has lived a double life: with various shore towns in Massachusetts and Rhode Island keeping the actual "parades of horribles" going, and lawyers and judges debating over figurative ones. Fortunately, I was able to get The Broccoli Horrible into the column under the wire, noting that it would make a pretty awesome band name.
[Update, 7/4: For further documentation, see my followup Word Routes column.]