My site consists of a sequence of form-submits. A very small number of browsers, less than 1% and perhaps only some IE browsers, will request the home page at the same time as every content page. With one of the problem browsers, typical section of IIS logs looks like this:

15-Mar-13   11:07:39 PM POST    /content.asp
15-Mar-13   11:07:39 PM GET /mainstyle.css
15-Mar-13   11:07:39 PM GET /images/logo.gif
15-Mar-13   11:07:39 PM GET /images/bar.gif
15-Mar-13   11:07:39 PM GET /images/fill.gif
15-Mar-13   11:07:39 PM GET /images/pbtop.gif
15-Mar-13   11:07:39 PM GET /images/hr.gif
15-Mar-13   11:07:39 PM GET /index.htm
15-Mar-13   11:07:39 PM GET /images/pbbottom.gif

The request to /index.htm is spurious (and causing a lot of trouble). (IIS logs "/index.htm" even when the request is just for "/".) Because the IIS logs are only resolved down to the full second, and items don't appear in exact sequence, it's hard to be sure if the strange extra request for index.htm is being triggered by the preceding page, or by something in the page that is loading.

My first thought was maybe there is a blank src="" in the page somewhere, but there isn't, at least not in any browser I use to test. It drives me nuts, because I can't duplicate it, and it only happens with a tiny fraction of users.

Has anyone ever seen anything like this, or any guess what could cause it?

Disable the "Content Advisor" in IE, via "Tools-> Internet Options-> Content Tab"...

If you don't have access to the button because the "Content Advisor" is protected by a password, you can try this: http://www.wikihow.com/Remove-Content-Advisor-Password-in-Internet-Explorer.

I had the very same issue and this solved it as I reported on http://social.msdn.microsoft.com/Forums/en-US/iewebdevelopment/thread/86c5130a-d4fa-471b-87b1-7bf51c36eb9e