Theme hook MENU_CALLBACK not found
In my case, this was a mistake on my part. I recently performed a find and replace on my codebase to change all my menu hooks from MENU_LOCAL_TASKS to MENU_CALLBACK.

I clearly did this ignoring case, and also did not check properly as in my template.php, I now had...

$vars['secondary_local_tasks'] = array(
    '#theme' =&gt; 'MENU_CALLBACK',
    '#secondary' =&gt; $vars['tabs']['#secondary'],
);


instead of...

$vars['secondary_local_tasks'] = array(
    '#theme' =&gt; 'menu_local_tasks',
    '#secondary' =&gt; $vars['tabs']['#secondary'],
);


Because it was now called MENU_CALLBACK in caps, it confused me and I was looking in the wrong place. It is only when I went back and diff'd my entire backed up codebase did I notice it.

Lesson Learned: Find &amp; Replace is not always a good idea :)