Problem with CiviMail Menu on Joomla 3.4.1 - CiviCRM 4.6.2
In my case the issue was the K2 component (version 2.6.9), Joomla 3.4.1 and CiviCRM 4.6.2, specifically its system plugin, which is required for K2 to function correctly. This plugin monkeys with the HTML  code and inserts Opengraph tags, but these are not escaped by CiviCRM, or they are inserted by K2 after CiviCRM has done its thing. 

See plugins/system/k2.php - specifically this code section:

if (JString::strpos($response, 'prefix="og: http://ogp.me/ns#"') === false) 
                    { 
                            $searches[] = '&lt;html '; 
                                    $searches[] = '&lt;html&gt;'; 
                            $replacements[] = '&lt;html prefix="og: http://ogp.me/ns#" '; 
                            $replacements[] = '&lt;html prefix="og: http://ogp.me/ns#"&gt;'; 
                    }


Change this to:

$app =&amp; JFactory::getApplication(); 
            if ($app-&gt;isSite()){ 

                    if (JString::strpos($response, 'prefix="og: http://ogp.me/ns#"') === false) 
                    { 
                            $searches[] = '&lt;html '; 
                                    $searches[] = '&lt;html&gt;'; 
                            $replacements[] = '&lt;html prefix="og: http://ogp.me/ns#" '; 
                            $replacements[] = '&lt;html prefix="og: http://ogp.me/ns#"&gt;'; 
                    } 
            } 


CiviRM bug tracker
JoomlaWorks / K2 issue report (and patch)