I tried to duplicate the behaviour (changing the post subject field during a preview) but couldn’t get it to work. Maybe a bug in vBulletin that happens at rare intervals, or else the server did an incomplete update again? Don’t know for sure.
Whoops! That was my fault. I merged the original thread (Deux Es Machina) with a reponse entitled “lal” which had mistakenly started a new thread. I just forget to check the title afterwards as it apparently took the title of the secondary thread. I’m changing it back now.