(S)omeone blocks (Y)ou. Web === If you are logged in as Y and go to http://twitter.com/S, you get no indication that you are blocked, being able to view S's timeline, etc.
API === When accessing S's timeline using /statuses/user_timeline/S.xml authenticated as Y, however, you get a 401/Not Authorized. A) Besides 401 having overloaded meanings (wrong credentials. it can also mean S has been deleted ), so we can't tell what is wrong, B) It is also inconsistent. Should Y be allowed to view S's timeline (web) or not (API)?  http://code.google.com/p/twitter-api/issues/detail?id=1226  Also reported at http://code.google.com/p/twitter-api/issues/detail?id=1243 -- Hwee-Boon