Yesterday our operations team pushed out a new version of Apache to
our cluster.  This new version no longer accepts invalid "Expect"
headers, which are apparently commonly sent by .NET applications.  A
workaround has been suggested by a developer here: 
http://bit.ly/9FsT.
Please ensure that your application sends this header correctly.
-- 
Alex Payne - API Lead, Twitter, Inc.
http://twitter.com/al3x