object/embed and Browser Based WYSIWYG HTML Editors
One problem I've run across with browser based WYSIWYG HTML editors is that they don't handle fallback HTML well. This problem is most evident when switching between the WYSIWYG view and the source view for an HTML document. I've primarily seen this issue with Internet Explorer when trying to include an object tag. In typical fallback fashion you include the embed tag within the object tag. The problem is when you then switch from source to WYSIWYG and then back to source, IE's internal DOM throws out the embed. The net result, a document that can only be edited in source view or a document that only works in Internet Explorer.
Granted maybe adding an object/embed tag through a WYSIWYG editor isn't the smartest thing, but the fact that information is being lost along the way doesn't give much credence to Microsoft's push for round-tripping. Maybe someone has solved this, but as best as I've been able to tell both FCKeditor and HTMLArea suffer from this problem.