If you create a post with a title (and slug) - I'll use
test as an example - and later create a page with the same title/slug, WordPress doesn't notice the collision and try to change the slug for you. If you try to create/change a post's slug to an existing post slug, it'll put a '-2' at the end. Same with a page. But WP doesn't seem to cross-check between posts and pages.
Is this a bug, or just the way it is (for whatever reason)?