If the user insert an Iframe in the Html mode of RadEditor, switch to Design mode and then switch back to Html mode, the HTML content becomes non-editable. The issue is reproducible in Chrome. Possible workaround: Use the RadEditor control in Div content mode: http://demos.telerik.com/aspnet-ajax/editor/examples/contentareamodediv/defaultcs.aspx
This bug is related to multiple browser issues, that are already fixed in Chrome. Upgrading the Chrome to Version 38.0.2125.101 m resolves the issue.
Using Div content mode is not always a viable workaround because of the way it treats CSS differently.