This iframe element has an empty html tag that can cause issues with the Section 508 validation.
Also, the iframe is not used so it does not need to be present in the markup at all.

The following script added to the OnClientShow event of the RadWindowManager can remove this iframe:
		function OnClientShow(sender, args)
		{
			if (!sender._isPredefined) return; //check if it is a predefined dialog
			var frame = $telerik.$("iframe", sender.get_popupElement());
			frame.remove();
		}

 Out of stack space error is thrown by the Find & Replace dialog in IE11/Edge when there is an img tag withing the content. The image will cause the Find feature to sit and spin when searching for a string after the img tag. It does not return a result and it does not trigger a modal to display, e.g., “The search string was not found.”

Changing the background color for the second/subsequent paragraph causes background color for first/previous paragraph to be lost/changed.  This is a browser-specific bug.

Reproduction steps:

    Navigate Internet Explorer/Edge browser to: Telerik RadEditor Demo Page
    Select-all and delete.
    With cursor at the beginning, select a background color from the toolbar.
    Type some text.
    Hit the Enter key to start a second paragraph
    Select the "no-fill" background color from the toolbar.

Results:

-The background color for the first/previous paragraph is removed.

Reproduction steps: 

When part of a table from MS Excel is copied and pasted into RadEditor under Edge, the RadEditor inserts an image instead of the html for the table. It inserts <img alt="" src="data:image/png;base64, ... " /> instead of the html.

Steps to reproduce:

Open http://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx in MS Edge.On the editor select the html and delete everything.
On the editor select design
Open any Excel file copy a couple of cells
Paste in the Editor
On the editor select html
Instead if html the there is an image tag
<img alt="" src="data:image/png;base64,iVBOR ... YII=" />

With firefox, IE 11 and Chrome the html is pasted. For MS Edge the Image tag is pasted.

Workaround:
<telerik:RadEditor runat="server" ID="RadEditor"></telerik:RadEditor>
<script>   
    Telerik.Web.UI.Editor.Utils.containsHtmlAtClipboard = function(oEvent) {
        var result = oEvent && oEvent.clipboardData && oEvent.clipboardData.getData &&
            oEvent.clipboardData.types && oEvent.clipboardData.types.length &&
            Telerik.Web.UI.Editor.Utils.containsElement(/text\//i, oEvent.clipboardData.types);
 
        return result;
    }
</script>

The empty new lines from RTF are imported as empty paragraphs in the editor and not rendered, i.e.

<p class="TelerikNormal" style="line-height: 16px;"> </p>
<p class="TelerikNormal" style="line-height: 16px;"> </p>
<p class="TelerikNormal" style="line-height: 16px;"> </p>
<p class="TelerikNormal" style="line-height: 16px;"> </p>
<p class="TelerikNormal" style="line-height: 16px;"> </p>


They should be rendered, because in other case this breaks the layout of the inserted document.

The context menu for IMG and Table element immediately hides when shown on Mac Firefox.

Load this demo http://demos.telerik.com/aspnet-ajax/editor/examples/overview/defaultcs.aspx on Firefox Mac OS
and right click in a table cell or image. You'll see how the menu pops up and immediately closes.

Video reproduction -> http://www.screencast.com/t/osHP96eQOa7

Due to that behavior the Editor control appears to change the actual content of the user, which is considered as a faulty behavior. 

When a parent control is disabled by setting Enabled to false, all children controls should inherit the behavior and act as disabled. RadEditor does not follow this logic and stays enabled. 

The exact steps to reproduce are:

1. Use RadEditor in http://localdemos.telerik.com/aspnet-ajax-dev/editor/examples/overview/defaultcs.aspx
2. Add this HTML:
        <p>start</p>
        <p>&nbsp;</p>
        <p>end</p>

3. Go to Design;
4. Select from the empty paragraph (<p>&nbsp;</p>) to the end of text (<p>end</p>);
5. Copy via Ctrl+C;
6. Try to paste at the end of the "end" paragraph.

When there are words highlighted as wrong and triggering the Finish Spellchecking without correcting any words, subsequent editing of words inside the sentence causes random cursor replacement. 

Workaround:

<telerik:RadEditor ID="RadEditor1" runat="server" OnClientLoad="OnClientLoad">
    <Content>Whether you need a mere Textbox with Google-like spellchecker, or a Word-like content authoring environment, the result is the same: clean <strong>XHTML</strong></Content>
</telerik:RadEditor>

<script>
    function OnClientLoad(sender, args) {
        sender.add_spellCheckLoaded(spellCheckLoaded)
    }

    function spellCheckLoaded(sender) {
        var editor = sender;
        var spell = editor.get_ajaxSpellCheck();

        spell.add_spellCheckEnd(function (sender, args) {
            editor.set_html(editor.get_html(true));
        });
    }
</script>

ImageManager loses the selection after an image has been edited into the ImageEditor and saved with new file name.

video: http://screencast.com/t/rZIp4iioehE

The height is bigger than the viewport.

If there are toolbars docked at the left or right, the right toolbar is hidden due to larger width. 


You can fix a bit the width by using this code:

<telerik:RadEditor ID="RadEditor1" runat="server" RenderMode="Lightweight" OnClientCommandExecuted="OnClientCommandExecuted">
    <Tools>
        <telerik:EditorToolGroup>
            <telerik:EditorTool Name="AjaxSpellCheck" />
        </telerik:EditorToolGroup>
        <telerik:EditorToolGroup dockingzone="Left">
            <telerik:EditorTool Name="AjaxSpellCheck" />
        </telerik:EditorToolGroup>
        <telerik:EditorToolGroup dockingzone="Right">
            <telerik:EditorTool Name="ToggleScreenMode"></telerik:EditorTool>
        </telerik:EditorToolGroup>
    </Tools>
</telerik:RadEditor>

<script>
    function OnClientCommandExecuted(editor, args) {
        if (args.get_commandName() === "ToggleScreenMode" && editor.isFullScreen()) {
            var editorId = editor.get_id();
            var editorElm = editor.get_element();
            var leftToolbar = $telerik.$(editorElm).find("#" + editorId + "Left.reToolBarWrapper");
            var rightToolbar = $telerik.$(editorElm).find("#" + editorId + "Right.reToolBarWrapper");

            var widthExcess = leftToolbar.width() + rightToolbar.width();

            if (widthExcess) {
                editorElm.style.width = (parseInt(editorElm.style.width) - widthExcess) + "px";
            }
        }
    }
</script>

This matter causes incorrect user interaction with the built-in tools.   

Due to the complexity of the IE11 changes of the selection object, with which the RadEditor control interacts to preserve the correct range the only possible resolution is adding a meta tag in the head element of the page to force the IE10 compatibility mode of the browser:

<meta http-equiv="X-UA-Compatible" content="IE=10">

Programmatically added EditorToolGroup to the RadEditor's Ribbonbar is not displayed when the control is localized in Lightweight rendering.

This issue can be reproduced when there is some initial HTML content entered that cause scroll to appear. Next, adding some new lines and having a mechanism to call the get_html(true) method would scroll the content area to a random position and cause flickering.

Having some text and input inside: Highlight it in order to copy and paste leads and the content area will scroll to the top of the content area. Also, you cannot highlight the input in order to copy and paste it. 

RadEditor in Fullscreen mode appears cut off at the right and at the bottom. 



In order to fix that, you can add this CSS rule to the page:

<style>
    .RadEditor.reFullScreen {
        box-sizing:border-box;
    }
</style>

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