Got it. Thanks for pointing me in the right direction.
For anyone else: I was re-labeling “Pages” and the pages submenu to something else using the pretty standard method found here: http://code.tutsplus.com/articles/customizing-your-wordpress-admin–wp-24941
However, that method doesn’t allow you to specify a submenu slug, causing the notice found above.
I ended up having to use remove_submenu_page() to remove those submenus entirely and add_submenu_page() to add them back with the labels that I wanted. No more notices! This technique might a bit convoluted, but it does work.