I test my webapps so rarely in IE that when they break, I assume it is IE's fault. I'm usually right. Tell me if this has happened to you: You're developing in an iframe situation where the master page is at a different domain than the inner page. This, by the way, is exactly the situation for iframed Facebook applications. You click on a link in your iframe, and then your frame blows up.
This is because IE's default "medium" privacy setting has an issue with cookies in that situation. So whatever session you setup for yourself on that first page doesn't exist when you try to click through to the next page of your application.
In my Rails controller I added this line:
which is a shortcut for "Before you do anything else, run the set_p3p method". And, the method:
response.headers["P3P"]='CP="CAO PSA OUR"'
Presto, it should work. Right? Well, no. At least, not for me. It stayed broken, and that's where I got stuck, because everything I googled said that should work.
Since that file had nothing to do with my ability to login to my application I'm not really sure how it caused the crash it did, but when I took out the include, magically everything began working. So perhaps IE has a policy that says "If any resources from this domain are blocked, then all are blocked."