I ran into this messing around with a fresh install of bbpress installed on WordPress 3.5. Basically if you follow these steps, you can cause a forum’s permalink to get stuck in a redirect loop:

Create a category
Create a forum
Make the category the parent of the forum
Trash the category without removing the forum as a child
Go to the forum edit page and update it with no parent selected

After doing this the permalink of the forum becomes an infinite redirect loop and you’ll get an error from your server and browser. When you get the error screen, your browser should show the old permalink that includes the category slug, even though the new permalink shouldn’t have it. The bad part is that even if you untrash the category and make it the parent of the forum again, the redirect loop stays the same. This also makes the slug of said forum unusable.

I did this by accident and then was able to reproduce using the previously mentioned steps. Just wanted to let you guys know. Can’t find a way to fix this but to prevent it from occurring, make sure you unlink forums from a category BEFORE you want to delete said category.

[@jaredatch Member 12 years ago]

Hmm, would you mind adding this to bbPress trac? https://bbpress.trac.wordpress.org/

[@funkatronic Participant 12 years ago]

Created the ticket: https://bbpress.trac.wordpress.org/ticket/2112

Lets hope it gets fixed.