londrum - 8:16 pm on Sep 13, 2011 (gmt 0)
do you sometimes cache the html/php output server-side before sending it to the user?
maybe you've got some kind of bug where it's saving a blank page, and then it serves that to the user. no error would show for that, because the error would have occurred beforehand, when you saved the page.
i would check wherever it is you cache your stuff, and see if you've got any 0byte files in there