that reminds me of this useless feature outlook/exchange has -- if you send out an e-mail and then change your mind, you can recall it. this has the effect of dropping what looks like another e-mail but with a slightly different 'unread' icon. when someone opens this fake e-mail, it just pops up a little box stating that the sender would like to recall their message. all you can do is click okay, and the original e-mail will still be sitting there.
Actually if you happen to open the Recall message while the mistake message is still unread they'll both be deleted once you click okay. But if you've already read the mistake message then nothing happens.
Of course since most people read their e-mail oldest to newest this almost never happens.