tl;dr: Posts whose net score ever becomes negative essentially vanish permanently due to a quirk in the algorithm. So an attacker can disappear posts he doesn't like by constantly watching the "New" page and downvoting them as soon as they appear.
And one of the reasons might be that sorting doesn't work quite properly on smaller subreddits?
http://www.reddit.com/r/programming/comments/1si31j/reddits_...