Hi there. I have just found a weird issue regarding the wp_redirections, I think that may be a WordPress weird thing but let me tell you the whole scenario.
I'm ussing WP 3.1 and the custom permalinks plugin, and the canonical.php file from the previous release... So, the custom permalink estructure like:
Works like a charm, but when you use characters like quotes on titles (and then in the url) wordpress handles a redirection to the parent (parent/postname) and the post is displayed correctly. So i managed to use the "sanitize urls" plugin and now my permalinks don't contains any weird character and there is no redirection.
But the weird thing comes here... When you use a number on any part of the post title like "2011 meeting" the structure breaks again and even if you make a duplicated post (and then a number is placed on the finish), the structure breaks again.
By doing debug on this, i get that the post names that contains numbers don't resolve category and fires the 404 error... The exactly same post, with removed number on post-name field on the sql works perfectly.
So, my guess is that WordPress is not doing the request properly, i't seems that any "post-with-12-numbers" post-name is being processed as a different query and is not the same thing that "post-without-numbers".
The plugin is not making this redirection because I have commented that action, I think that is just a WordPress weird behavior.
Right know I'm trying to find where WordPress reads the request and process them in order to act like this. Using 3.0.5 was the same thing so i'm assuming that this is not a 3.1 issue, maybe this works on 2.9 (i haven't test on that release).