Tweets in my Tweetbot timeline are occasionally marked as a favorite when I see them for the first time. Is that a bug or a feature? While looking through my timeline in Tweetbot, occasionally a tweet will be marked as a favorite even though I'm sure I'm seeing it for the first time.  (I can't resist taking the time to unfavorite it.)  I don't see any mention of this in http://tapbots.com/support/tweetbot/.  Is this a bug, or a "feature" (where perhaps Twitter's API is doing this)?
It's a bug with Twitter, not Tweetbot.
I've seen people ask @tweetbot about it several times now. The answer and response generally look like this exchange:

Q: @tweetbot keeps favoriting random tweets in my timeline. It's almost embarrassing.
A: Sorry, that's a Twitter bug. As you can see on Favstar, they're not real favorites.

That particular exchange happened just yesterday.
Twitter's REST API bug list has this bug still open where the API may erroneously report a tweet as favorited when in fact it is not.