Someone discovered a bug today where under certain circumstances I show a follow button even if you're already following a user. Naturally the person hit the follow button, because they wanted to follow the person.
This in turn sent out 'follow' events on user streams and site streams. (I'm not sure about emails - didn't test them). This resulted in an awkward conversation for the two users.
To protect against buggy apps (mine) and malicious apps, I think it would be best if Twitter didn't send a "new follower" event on the streams if the followship already exists.
Updates
Additional data point from @timhaines:
"Okay - I just reproduced it. It seems like it only happens if the followship is reasonably aged. Can't do it with a new follower 2x."