class Twitter::InternalServerError

Runtime error leaf class raised when Twitter.com API is borked for an unknown reason. HTTP code: 500 (aka Internal Server Error).

To handle specifically you would do the following:

begin
  timeline = twitter.timeline_for(:friends, :since => tweet)
rescue InternalServerError => ise
  # do something to notify user that an unknown internal server error
  # has arisen.
end