This is an issue in several different browsers, but most notably under IE and variants. A large part of the issue is that, under the default FCK configuration, radius tags are not protected. This means that when a page including radius tags is loaded or saved, the data on the page gets parsed through a XML DOM tree. Depending on the browser and XML parser that you're using, this can lead to everything from tag corruption to javascript execution crashes. This is most notable when using deep radiant tags, like <r:children:each></r:children:each>, since most XML parsers won't correctly translate that into <r:children><r:each></r:each></r:children>.

A good workaround is to add this line to /public/javascripts/fckconfig.js :

FCKConfig.ProtectedSource.Add( /<\/?r:[^>]*>/gi );

This will cause radius tags (ie, any tag beginning with <r: or </r: ) to be classed as protected under FCKeditor -- they won't appear in the editor window, nor will they be passed to the XML parser. Radius tags can still be viewed and modified by turning off the FCKeditor filter.

I've verified that this works under FireFox, Chrome, and IE8. Has anyone else had any luck?