via Sophos (and they found it via reddit)

If you try to change set your password to “correcthorsebatterystaple” at dropbox, you’ll be advised that you shouldn’t take password advice quite so literally.   See the Sophos link for screenshots.

XKCD had a cartoon about passphrase generation.  I recall at the time predictions that people would use the phrase used in the example.   Rather than being evidence of a bad password dictionary, this is a easter egg put in place by the web developer.   One that likely wont translate well, as noted in the Password_Strength.js javascript.