menu_link_save returns mlid even when the menu item is not saved

Can confirm the issue.

menu_link_save() function needs a valid menu item.

with the menu_form the new menu item path ($item['link_path'])
is validated with function menu_edit_item_validate()

if (!trim($item['link_path']) || !menu_valid_path($item)) {
    form_set_error('link_path', t("The path '@link_path' is either invalid or you do not have access to it.", array('@link_path' => $item['link_path'])));
  }

see: http://api.drupal.org/api/function/menu_edit_item_validate/6

if you call menu_link_save() directly you need a valid menu item.
i think its not a good solution to check valid paths twice. once at menu_edit_item_validate()
and once at menu_link_save().

That assumes that menu_link_save() isn't being invoked programmatically, outside of the normal use case in which it would already be validated first. While I agree that double-handling should be avoided where it can be helped, I don't know if that's the case here. Reading the documentation, it says nothing about the params of menu_link_save() requiring validation before being passed in, and claims that FALSE is returned if the item didn't save. If it isn't returning FALSE on failure, it's a bug.

Following to see where this goes. I agree with c.ex after stumbling to debug a similar issue.

This still appears to be an issue in D7. I'm not sure whether the bug is in the documentation ("The mlid of the saved menu link, or FALSE if the menu link could not be saved due to the maximum depth being exceeded," which doesn't make as much sense) or the code (not returning FALSE when failing to save for other reasons), but one of these is clearly incorrect. Any ideas?

Bumping this, menu_link_save shouldn't return mlid if the call is not successful.