[chrishajer]

WordPress is thinking this forum page does not exist., so WordPress throws the 404 but the page is still properly served. It happens only because you’ve included blog-header.php (a normal integrated installation wouldn’t do this).

I think this post covers it (basically, install a WordPress plugin to overcome the problem):

https://bbpress.org/forums/topic/bbpress-wordpress-mu-or-not-leads-to-404-errors-but-pages-still-load#post-13156