Updated, see below.
A search for “People I don’t follow” and “People they don’t follow” reveals that a large number of users are suffering the problem. It appears to have been going on since at least yesterday, when celebrity blogger Perez Hilton complained of the apparent glitch.
Some of these users are clarifying that the issue is nothing to do with retweets. Ironically, I was tipped off about the problem by my father who saw a tweet from someone he didn’t follow. In the tweet, the user was complaining about that very problem.
Yesterday Twitter had a similar problem with replies. The new glitch appears to be general tweets getting mixed up somewhere in the Twitter matrix. We’ll reach out to Twitter for comment and let you know if/when we hear more.