I did not have this problem before I upgraded to 2.5.1.
Right now, all of my pages that have a slug that is made only of numbers cannot be accessed and redirect back to their parent Page.
For example, I cannot access 2007 which is a subcategory of articles:
I get the first page, no matter what I do. The second page, of course, exists in the admin panel and there was no redirection set.