The bug has been opened on https://bugs.launchpad.net/ubuntu/+source/telepathy-butterfly/+bug/561050

"I just saw this crash report 

telepathy-butterfly 0.5.8-1

Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/papyon/switchboard_manager.py", line 75, in <lambda>
    lambda sb, contact: self.__on_user_joined(contact))
  File "/usr/lib/pymodules/python2.6/papyon/switchboard_manager.py", line 136, in __on_user_joined
    self._on_contact_joined(contact)
  File "/usr/lib/pymodules/python2.6/papyon/conversation.py", line 335, in _on_contact_joined
    self._dispatch("on_conversation_user_joined", contact)
  File "/usr/lib/pymodules/python2.6/papyon/event/__init__.py", line 44, in _dispatch
    if event_handler._dispatch_event(name, *args):
  File "/usr/lib/pymodules/python2.6/papyon/event/__init__.py", line 65, in _dispatch_event
    handler(*params)
  File "/usr/lib/python2.6/dist-packages/butterfly/channel/im.py", line 154, in on_conversation_user_joined
    signal=True, conversation=None)
  File "/usr/lib/python2.6/dist-packages/telepathy/server/channelmanager.py", line 120, in channel_for_props
    return self.create_channel_for_props(props, signal, **args)
  File "/usr/lib/python2.6/dist-packages/telepathy/server/channelmanager.py", line 105, in create_channel_for_props
    props, **args)
  File "/usr/lib/python2.6/dist-packages/butterfly/channel_manager.py", line 157, in _get_text_channel
    object_path=path)
  File "/usr/lib/python2.6/dist-packages/butterfly/channel/conference.py", line 62, in __init__
    self._invite_initial_invitees(props, ic)
  File "/usr/lib/python2.6/dist-packages/butterfly/channel/conference.py", line 147, in _invite_initial_invitees
    raise telepathy.NotAvailable('Contact "%s" not available' % invitee_id)
NotAvailable: org.freedesktop.Telepathy.Error.NotAvailable: Contact "n" not available"