I went to open the same local site I was working on in both FireFox and IE to see the differences in their appearance, and when I tried to access the site in IE7, I got:
HTTP 403.9 - Access Forbidden: Too many users are connected
Hang on, on LOCALHOST!? I've always got multiple browsers accessing local sites and had never encountered this problem before, but it intrigued me.
I have had numerous instances of Visual Studio's development server opening and closing over the last few days, and many times, running in debug mode. Sometimes, these servers have crashed and have had to be manually exited via task manager. It's my belief that this has something to do with my local IIS thinking there are multiple users attached.
I can confirm that, by default, your local IIS will only allow ten connections. No problem with that, until you end up with nine or ten ghost connections.
It's quite probable that the "HTTP Keep-Alive enabled" setting (which is necessary for running your Visual Studio projects in debug mode) has something to do with the issue, particularly in conjuction with in-debug crashes. However, whatever the issue, I found a simple IIS restart fixed everything up right away.