I found this problem when trying to load a URL which runs a web based file manager application called Pydio (formerly AjaXplorer).
Any Pydio installation doesn't work on IETester even though the site loads properly on real IE and other browsers. Some IE version tabs behave a bit differently though.

You can see a demo of Pydio at https://demo.ajaxplorer.info/ for testing

IE5.5, IE6 and Default IE tabs (with IE 10 installed) only show a small XML tree with this content

<?xml version="1.0" encoding="UTF-8" ?><tree><require_auth /></tree>

IE7 and IE8 tabs both load and display some kind of real page layout (though both look different). The app still doesn't work because it seems to throw a lot of JS errors on IETester but at least you see the page and can even login.

IE9 tab shows an empty page but if you view page source you see the same XML tree as for IE5.5, IE6 and IE10 (Default IE) tabs.

I tested this with IETester 0.5.2 on Windows 7 Home Premium (64 bit).