I just installed the Error 404 plugin for use with ELGG 0.9RC1.
Instead of outputting the contents of the error message template, by default it output the name of the file. I was able to work around this by changing the call to the templates_page_draw function in mod/error404/lib/function_notfound.php as follows:
echo templates_page_draw( array(
sitename . ' :: ' . __gettext('404 Page not found...'),templates_draw(array(
'context' => 'contentholder',
'title' => __gettext('404 Page not found...'),'body' => file_get_contents($body)
))
));