Twitter has a reputation for occasionally “unfollowing” people for you without your permission. I don’t know if this is true. Maybe I don’t keep close enough tabs on my “followed” list. I do know that it this “bug” (or the rumor of it) is a brilliant feature.
It gives us an out in the awkward social situation when we called out for unfollowing someone. We can blame it on Twitter. Twitter don’t care.
Personally, I think the phenomenon exists. The best way to get the social dynamic established is to have an actual “bug” in the system. I just don’t think it is a mistake.
The Finch and Pea 
I’ve never noticed this myself. I do sometimes think I’m following someone, but then realize I’m not. I just saw them retweeted a lot, or I was following them from another account. But that’s no bug, that’s just me not paying attention.
That happens to me a lot. I thought I was following Ed Yong for a year before I was actually following him.