Currently, if $wgCookieSetOnAutoblock is set to true and someone gets a block cookie set, but then $wgCookieSetOnAutoblock is set to false (and the user's cookie still exists), they will get a fatal error when they try to edit a page:
Fatal error: Call to a member function setCookie() on a non-object in /Users/kaldari/Sites/core/includes/user/User.php on line 1665
This is because it's trying to call setCookie() on $block (to clear it), but $block isn't an instantiated object in this case.