Update at the bottom of the post.
Chances are that it’s a quick glitch. Twitter already knows about it, at least:
We’ve contacted Twitter, and we’re watching the Twitter Status blog, so we’ll let you know when things are sorted.
Update 2: It appears that Twitter is still having issues as we’re getting more reports of missing Follower/Following counts. The Support account has said that the List and Tweets counts have returned to normal, but this new problem just popped onto the radar.