Inspired by the Google Labs Ngram suggesting that we've reached peak future, I decided to map the term "unintended consequences," and for good measure "unanticipated consequences." I've been interested in the history of unintended consequences for a while (here's a PDF of an article I've written on the subject), and found references in Poole's and other newspaper and magazine databases going back to the mid-1800s, but hadn't done a similar search for books.
Here are the rather unsurprising results ("unintended consequence" is the line in red, "unanticipated consequence" in blue):
It's hard to see, but the line is pretty flat until World War II. From that point "unintended consequence" takes off, and "unanticipated consequence" rises more slowly but still substantially. You can see a bigger version here.
One day I'll have to take on self-fulfilling prophecy. (Actually, that one has a really curious trajectory.)