Any drupal version I run on my host (tried 4.6.10, 4.7.6 and 5.1) only works up to about 10-12 clean URLs in the db and after that it deletes existing aliases when I try creating new ones. Before that it won't create the aliases at all so I have to manually add them in the db(through phpmyadmin). If I add them in the db(phpmyadmin) after, they work.
Like many consulting firms, the group I work with (Openflows) maintains a number of older installations of Drupal which we can't upgrade for various reasons, but which we still need to keep stable and secure.