I'm not sure if this is a reliable source but have a look at this example: http://redbot.org/?uri=http%3A%2F%2Fwww.knightsenglish.com
i.e.
- The Pragma header is being used in an undefined way.
HTTP only defines Pragma: no-cache; other uses of this header are deprecated. - The Etag header's syntax isn't valid.
The value for this header doesn't conform to its specified syntax; see its definition for more information.