Can be reproduced with the following page setup:
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="Default" %>
<%@ Register Assembly="Telerik.Web.UI" Namespace="Telerik.Web.UI" TagPrefix="telerik" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<telerik:RadScriptManager ID="RadScriptManager1" runat="server">
</telerik:RadScriptManager>
<div style="height:930px">
Scroll down to bottom of the page and right mouse click to see context menu miss placement....
</div>
<telerik:RadEditor RenderMode="Classic" ID="redHtmlEditor" runat="server"
ContentAreaMode="Div">
<Content>
Right-click on image to see custom menu
<img style="marign-left: 6px" src="http://demos.telerik.com/aspnet-ajax/Editor/Img/productLogoLight.gif" />
<br/><br/>
<p>Right-click here to see the custom Paragraph menu.<br/></p><br/>
Right-click on the link to see the default link menu disabled: <a href="/">www.telerik.com</a>
</Content>
</telerik:RadEditor>
</form>
</body>
</html>