I just recently updated to WordPress 3.5.2 and found myself with random 500 server errors when accessing the backend. It took quite a while to debug (after disabling all plugins, checking .htaccess & deactivating/uninstalling W3 Total Cache) and this is what I found...
PHP Fatal error: Call to undefined function wp_get_current_user() in /var/www/www.MY-WEBSITE.info/htdocs/wp-includes/post.php on line 2121
We are on a multi-server setup which might explain the random occurences of the errors. It seems the plugin tries to execute/check wp_get_current_user which isn't loaded/available yet.
After disabling Plugin Organizer it all worked again which leads me to believe that it is a plugin issue which started with the 3.5.2 update.
Any info on why this happens / a workaround?