"I found a recent post in a fairly inactive subreddit and downvoted it, bringing its total vote score negative. Sure enough, that post not only dropped off the first page (a first page which contained month-old submissions), but it was effectively banished from the “Hot” ranking entirely. I felt bad and removed my downvote, but that post never really recovered...
While testing, I noticed a number of odd phenomena surounding Reddit’s vote scores. Scores would often fluctuate each time I refreshed the page, even on old posts in low-activity subreddits. I suspect they have something more going on, perhaps at the infrastructure level – a load balancer, perhaps, or caching issues."
This is partially due to vote fuzzing. More to the point, votes go into a queue and the removal of the downvote might not cancel out the previous action for some time.
As a result, this suggested flaw will supposedly let somebody successfully snipe puffins from the new page of a small sized birdwatching subreddit before they ever get a fair shake. I think if somebody would attempt this sort of manipulation further they would find it an ineffective strategy, there have been (probably constantly are) attempts to game Reddit before and this seems like an excellent honeypot.
Beyond the narrow set of circumstances during a very small time window the flaw disappears, yet if you try to abuse this you'll stick out like a sore thumb.
The true horror expressed in the OP is that the ordering of posts in the purgatory is not strictly logical - the post ranked 10042 should really be ranked 10041. Gasp. Twitch.
This is a very lovable brand of OCD to my eyes. :)