IE Mixed Content Warining when using https URLs and http:443 URLs?

I'm getting the good ole' "This page contains both secure and nonsecure items." dialog in IE when connecting to an HTTPS site. No big deal... I've just got something coming in over a non-secure connection so that should be an easy fix, right?

So I go into "View > Web Page Privacy Policy..." to look to see where I've included开发者_如何转开发 an HTTP file, and this is what I see...

https://blah/path/to/file.htm
https://blah/path/to/file.js
http://blah:443/path/to/file.css

Um... ok... so... there is an HTTP only URL being requested, but it is going over port 443 ("https://blah/" is shorthand for "http://blah:443/" EDIT: No it's not, actually [see slugster's answer/my comments below]) so...

What is the deal with this!? IE 7.0.5730.13 can't possibly be THAT stupid, can it?

Is there an IIS setting that needs to be tweaked?

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You've specified the port, but you have also specified the protocol as normal (non-secure) http. It isn't IE being stupid - you are expecting it to cover for your mistake.

This is normally the result of using an absolute path somewhere in your code instead of a relative path.