The page is published and using the default template.
Here's the code that calls out the pages for the navigation bar:
<?php wp_list_pages('title_li=&include=12,8,9,10,11,2,13,77&sort_column=menu_order') ?>
As far as my structure being non-standard, well, it is a bit page-centric. I did this because I couldn't get posts to organize heirarchically. But the more I think about this problem, maybe wp_list_pages isn't the culprit here. After all, it successfully puts page 77 into the navigation bar, and does attempt to go to the page. It's just not finding it. So...I'll ask again: is it possible that I did something to the setting of that particular page that renders its contents inaccessible to wordpress? Something I might have done in order to all posts (not pages) within it?