Class Twitter::NotAcceptableError
In: lib/twitter/core.rb
Parent: RESTError

Runtime error leaf class raised when the format specified in the request is not understood by the Twitter.com API. HTTP code: 406 (aka Not Acceptable).

To handle specifically you would do the following:

 begin
   timeline = twitter.timeline_for(:friends, :since => tweet)
 rescue NotAcceptableError => nae
   #
 end

[Validate]