If you watchlist a page with a name beginning with "Talk:Foo:", where "Foo" is a
namespace prefix such as "User" or "Image", the page cannot be unwatched via
Special:Watchlist/edit, since it will be listed without the "Talk:" prefix.
Such pages are occasionally created, usually by mistake, and may end up in the
watchlists of new page patrollers.
Simply including the "Talk:" prefix in the checkbox value attribute in such
cases would be sufficient to fix the problem.
http://bugzilla.wikimedia.org/show_bug.cgi?id=2871 is related to this bug, but
neither depends on nor blocks it.