Putting an SVG element in the editor content will throw an exception under IE.

Workaround (Make sure to add the script element with the fix right after the RadEditor declaration):

<telerik:RadEditor runat="server" ID="RadEditor1">
    <Content>
        <svg height="100" width="100">
          <circle cx="50" cy="50" r="40" stroke="black" stroke-width="3" fill="red" />
        </svg>
    </Content>
</telerik:RadEditor>

<script type="text/javascript">
    var oldMarkListItems = Telerik.Web.UI.Editor.WordListConverter.prototype.markListItems;

    Telerik.Web.UI.Editor.WordListConverter.prototype.markListItems = function(htmlText)
    {
        if (!htmlText)
            return htmlText;

        return oldMarkListItems.call(this, htmlText);
    };
 </script>