If you can't reproduce it reliably and it's kind of rare... isn't that enough to explain it? :P First thing you need in a bug report is a reliable way to get it to happen!

Also having read Raymond Chen's "Old New Thing" blog for a while, it's very common that some third party software/extension does something insane with the desktop window and breaks things. It could be something like that. Can you reproduce it on a completely empty fresh install of Windows?