Google’s HTTP replies have headers like this in them:
The first one happens for responses with result code 200. The second one happens with responses of 403. (If you are asking yourself how I got Google to give me a 403, try http://google.com/wml with a user agent that does not include “Mozilla” in it. Weird.)
Google is too smart of a company to do this by mistake. But what crazy bug could they possibly be working around with this?