I don't know how I missed this. It's been set up like this for a LOOOONG time and I never noticed.
I have my gallery in .com/gallery with my forums in the root folder. On the navbar, I have a link to the gallery (gallery/index.php).
Once a user is in the gallery, if you click on the link in the navbar, it points to gallery/gallery/index.php, which of course doesn't exist.
How would I go about fixing that?