We started looking at our IIS logs trying to figure out some 500 errors, once fixed we started with other improvements.
We found that a very old image backgroud file was still being referenced, thousands of times, but had been removed, many months ago, generating thousands of 404 entries. We found by adding to the logs "Referer" we were able to find the file that was referencing the file. Once fixed we saw a quick decreese in these failed requests.
Now 2 months after we still see this file being requested, mabey a couple of hundred times a day, the strange thing is that the refereer is still the same file that was updated months ago. We've done many sudgested fixes, including modifying the referer file several times, and updating response headers for the "referer" page. None have worked.
Now think that this is caused by some browsers not under our control have "check for newer versions of stored pages" "never", this is causing the "referer" page to never being updated, thus, it continues to try and download the broken 404 image file. I have no idea as to how to go about fixing this issue.