When you submit the page, the browser collects the values of all input and textarea elements on that page and send them to the server. In HTML mode the editor's content is a textarea which means that it is send to the server in every page postback .
"A potentially dangerous Request.Form value was detected from the client." exception is thrown every time when the ASP.NET framework detects HTML in a form element's value. The best solution is use the Rad Editor's Content Property
on the server to get and modify the content. You can then use the Editor's "ContentEncoder" to modify it.
There's a very useful Encoding the Html on the client
forum thread that provides an example on doing this.
Hope this helps!