I have one bad behaviour with post slug. I write in russian and thus i have titles mostly in russian like "пример заголовка". So i use slug to set more web-friendly permalink (something like "test-post"). And if i edit that post and didn't change title and forget to explicitly set slug - WP changes old slug to new one, in russian!, "пример-заголовка". And its changes post permalink. Here are some examples:
Lets say i have a post and want to edit it (using TextMate).
I fetch it and have something like this
Type: Blog Post (Markdown) Blog: NilColor@ Link: http://nilcolor.../2007/10/29/vacation/ Post: 82 Title: Отпуск
now if i add some info to the post body and save it i get this
Type: Blog Post (Markdown) Blog: NilColor@ Link: http://nilcolor.../2007/10/29/%d0%be%d1%82%d0%bf%d1%83%d1%81%d0%ba/ Post: 82 Title: Отпуск
but if i explicitly set
Slug: vacation than post permalink stays unchanged... Why WP recalc slug if i didn't change post title (and no slug sent to server)?