Ive come across a site whose doing this in .htaccess
<FilesMatch "\.(ico|pdf|flv|jpg|jpeg|png|gif|swf|mp3|mp4)$">
Header set Cache-Control "public"
Header set Expires "Thu, 15 Nov 2010 20:00:00 GMT"
Header unset Last-Modified
</FilesMatch>
#2 hours
<FilesMatch "\.(html|htm|xml|txt|xsl)$">
Header set Cache-Control "max-age=7200, must-revalidate"
</FilesMatch>
<FilesMatch "\.(js|css)$">
SetOutputFilter DEFLATE
Header set Expires "Thu, 15 Nov 2010 20:00:00 GMT"
</FilesMatch>
Why would anyone do that is beyond me. but… webpagetest.org gives this site an A in caching… In the same time the repeat view does a if-modified-since request for this.
Would have been easier to spot this mixup if WPT gave negative scoring for such things… small cache durations already result in a warning… negative duration should be an err
-Sajal