I nearly caused a minor kerfluffle over a misunderstanding. I received notification from twitter that @Scobleizer had followed me on twitter, whom I admire and had been following for a long time. I clicked the link to see... but he wasn't at the top of my followers list, and I was not in the first few pages of his following list.
 
  "He followed and immediately unfollowed," I thought. Unfollowing isn't a big deal, you unfollow when your interests change. A little while ago Scobleizer mass-unfollowed 106,000 people to simplify his feed, and I was among the mass-unfollows at that time. That wasn't a big deal.
  This seemed different as it was a more personal judgement: follow, decide that I am uninteresting, unfollow. That kind of judgement could have been done invisibly by reading my feed, and I never would have known. Following just to unfollow seemed rude, and I stewed over it. Why did he do that?
 
  The explanation is much more mundane: he didn't unfollow. As I said Scobleizer had followed me a long time ago, then mass-unfollowed. When he started following again he didn't appear at the top of the followers list, he appeared way back where he was when he started following the first time. Similarly if you go way, way, way back in http://twitter.com/scobleizer/following you will eventually find me.
 
  When you unfollow someone it appears that twitter never forgets the association, they just mark it as deleted. If they follow you again they reactivate the original follow. So the entry is not being inserted at the top of the list, it remains where it was the whole time. As @Scobleizer re-follows more and more people, this might come up again and again. "Why did he immediately unfollow?"
 
  I suspect Twitter does this deliberately. If an unfollow+refollow was sufficient to move you back up to the top of the list of followers, the bots would do it all the time to make it more likely you'd follow them back.

  There is also a more technical rambling on this topic at Coding Relic.