OK, I understand what the messages means, but I\'m really not sure what\'s causing it. I\'m using Safari and the Web Inspector on Mac OS X, by the way.
This bug seem to have resurfaced (Noticed it November 2010)
I think the WebKit bug reports involved are this one and this. Essentially it boils down to incorrect cache handling when doing an If-Modified-Since which get a 304 response.