When opening a ModalWindow inside a ModalWindow, the inner ModalWindow generates a 404 error. Both windows use a PageCreator for content.
To replicate, you must use an actual IE 7 or IE 8 browser, as this does not replicate using developer tools and setting the document and brower to IE 7.
The problem can be seen at http://www.wicket-library.com/wicket-examples/ajax/modal-window. I will attach a Quickstart as well.