Slug is redirecting to 404

This was caused by the Redirection plugin. Apparently it is trying to help by setting up redirects when you change the slugs for pages/posts. You have to click on “Groups” at the top of the Redirection plugin settings and then choose “Modified posts” to see the redirection that was automatically created to point from the old slug to the new one. Unfortunately these are not cleaned up when pages/posts are deleted.