You can workaround this issue by using this override:

<telerik:RadEditor runat="server" ID="RadEditor1">
    <Content>
        <br/>
        <iframe src="about:blank">Iframe not supported</iframe>
    </Content>
</telerik:RadEditor>

<script>
    Telerik.Web.UI.Editor.Utils._copyElementsBetweenNodes = function (oldNode, newNode, elementName)
    {
        var oldElements = oldNode.getElementsByTagName(elementName);
        var newElements = newNode.getElementsByTagName(elementName);
        for (var i = 0; i < oldElements.length; i++)
        {
            switch (elementName)
            {
                case "script":
                case "title":
                    newElements[i].text = oldElements[i].text;
                    break;
                case "style":
                    if (newElements[i].innerHTML != oldElements[i].innerHTML)
                    {
                        newElements[i].styleSheet.cssText = oldElements[i].styleSheet.cssText;
                    }
                    break;
                case "select":
                    newElements[i].selectedIndex = oldElements[i].selectedIndex;
                    break;
                case "input":
                    if ((oldElements[i].type == 'radio' || oldElements[i].type == 'checkbox') && $telerik.$(oldElements[i]).attr("checked"))
                    {
                        $telerik.$(newElements[i]).attr("checked", "checked");
                        if ($telerik.isIE9Mode && oldElements[i].type == 'checkbox' && oldElements[i].checked)
                        {
                            newElements[i].checked = true;
                            newElements[i].setAttribute("checked", "checked");
                        }
                    }
                    break;
                case "iframe":
                    var innerHTML = oldElements[i].innerHTML;
                    if (innerHTML)
                    {
                        var outerHTML = newElements[i].outerHTML;
                        if (outerHTML)
                        {
                            var gtIndex = outerHTML.indexOf(">");
                            if (gtIndex > 0)
                            {
                                outerHTML = outerHTML.substr(0, gtIndex) + ">" + innerHTML + outerHTML.substr(gtIndex + 1 + innerHTML.length);
                                newElements[i].outerHTML = outerHTML;
                            }
                        }
                    }
                    break;
                default:
                    if (newElements[i].innerHTML != oldElements[i].innerHTML)
                        $E.Utils.setElementInnerHtml(newElements[i], oldElements[i].innerHTML);
                    break;
            }
        }
    };
</script>

Inserting an image after opening ImageManager from within the ImageManager's Properties tab throws JS error. The issue is reproducible both in IE and Firefox

video: http://screencast.com/t/n4T0XYtSpt

For example, using this markup:

<telerik:RadEditor ID="RadEditor1" runat="server" ToolbarMode="RibbonBar">
</telerik:RadEditor>

and this Page_Init code:

protected void Page_Init(object sender, EventArgs e)
{
    RadEditor1.RenderMode = RenderMode.Lightweight;
}

Causes icons to either miss or their position is incorrect.

Instead of null, the String.Empty can be used. 

When user has changed image properties via Image Properties dialog, changes are getting cleared when area is being added via Image Map Editor. 

For the time being you can disable the StripCssExpressions  filter:
    protected void Page_Load(object sender, EventArgs e)
    {
		RadEditor1.DisableFilter(Telerik.Web.UI.EditorFilters.StripCssExpressions);
    }

Alternative approach to fix the issue is removing the StripCssExpressions filter from the client-side filters collection, so that the server-side functionality could still sanitize the HTML content:

<telerik:RadEditor runat="server" ID="RadEditor1" OnClientInit="OnClientInit">
	
</telerik:RadEditor>

<script type="text/javascript">
	function OnClientInit(sender, args) {
		sender.set_contentFilters(sender.get_contentFilters() - Telerik.Web.UI.EditorFilters.StripCssExpressions);
	}
</script>

When the EnableTrackCahnges functionality of the Editor is set to true, a context menu is shown only if the user right-clicks over a tracked change. Clicking over a not changed element does not pop-up any context menu (neither the Editor's not the browser one). The issue is reproducible in all browsers.

Steps to reproduce:
1. Run the following configuration:

            <telerik:RadEditor ID="RadEditor1" runat="server" EnableTrackChanges="True">
                <Content>
                    <span>SPAN dssad</span>
                    <div>DIV fsdfksjdfkf</div>
                    <p>Paragraph jfdshfksjdhfksdhk</p>
                </Content>
            </telerik:RadEditor>

2. Right-click over a not track-changed element.

Actual result: No context menu is shown.

Expected result: The Editor's default content menu (or the browser on) is shown.

When a tool has a longer name, part of the text drops to a new line, and causes incorrect layout. 

To resolve you can use the following CSS rule:

.reDropDownBody a.reTool.reToolText {
    white-space: nowrap;
}

When pasting list items over an existing one, the list from the clipboard is added to new UL element. The proper behavior it should be to paste the existing content and adapt it to the existing list.

When EnableTrackChanges property of RadEditor is set to true:

-The font name of the written text in RadEditor cannot be change and instead the default one is used. This behavior is observed under FireFox, Chrome and Safari. The workaround is to mark the already written text and then set the desired font.

-The chosen font size from the toolbar returns to its default value when the cursor changes its position and then returns back to its previous position. This behavior is observed under FireFox.

Set the NewLineMode to br tag. Then inserting a list and closing it appends the following:

<p>&nbsp;</p>

It should be appending a <br> element corresponding to the set mode.

Using an MS Word file with an inline image or an carriage return (<br/>) to paste the content in the editor, the ConvertWordList StripFormatting option removes the <img /> and <br /> elements. 

When user selects an image in the RadEditor resize handles appear. When resizing or replacing the image, the same handles still appear at the initial position. This confuse the users that the image is not selected and leave an incorrect trace of the resize handles.