Roy Hodson pointed out to me that a recent article by Larry Dignan on "The economics of Microsoft's kill switch" said:
A behind-the-envelope calculation illustrates why it makes sense for Microsoft to risk irking techies with its piracy battle.
Not quite clear whether Larry meant behind the veil, behind the curve, back of the envelope, pushing the envelope, back of the curve, behind the woodshed, back of the veil, pushing the veil, back of the woodshed, pushing the woodshed, or pushing the curve, is it?
A Google check suggests that Larry may be the only person ever to have used the phrase "behind the envelope calculation" in the history of the world. If I had found even one other occurrence, then under the OICTIQ principle I might have considered the possibility that we have a new idiom emerging here; but I think not. I'd say we're simply looking at a one-off mistake due to a confusion between two idioms. A sort of phrasal malapropism. And if you are surprised that a linguist would think a native speaker can make a mistake about the use of his own language, you shouldn't be.
Although, of course, we shouldn't forget that this is the kind of error that can sometimes act as a little seed from which a legitimate linguistic change might one day grow.
[Update: Dave Errington has made the very sensible suggestion that Dignan might have taken "back of the envelope" to relate to the phrase "in back of the envelope", meaning "behind the envelope" (as opposed to "on the back of the envelope"), and thus replaced the former by the latter either as a confusion or because he saw the two as synonymous. And John Cowan suggests that there might even have been an editorial intrusion here — a general substitution of behind for (in) back of, carelessly over-applied to a case that meant "on the back of"; there were a few (misguided) 20th-century usage handbooks that followed the opinionated grouchiness of Ambrose Bierce (Write It Right, 1909) and called (in) back of an illiteratism, for seventy or eighty years. (There is in fact nothing wrong with the phrase, though it is distinctively American rather than British.)]
Posted by Geoffrey K. Pullum at November 22, 2006 01:02 PM