This was originally reported as a Red Hat bug 586228:
https://bugzilla.redhat.com/show_bug.cgi?id=586228
With 0.5.12, the traceback is slightly different to what is present there:

connection_manager.py:87:quit:TypeError: 'set' object is unsubscriptable

Traceback (most recent call last):
  File "/usr/lib/python2.6/site-packages/butterfly/util/decorator.py", line 87, in async_function
    func(*args, **kwargs)
  File "/usr/libexec/telepathy-butterfly", line 67, in quit
    manager.quit()
  File "/usr/lib/python2.6/site-packages/butterfly/connection_manager.py", line 87, in quit
    conns = self._connections[:]
TypeError: 'set' object is unsubscriptable

Local variables in innermost frame:
self: <butterfly.connection_manager.ButterflyConnectionManager at /org/freedesktop/Telepathy/ConnectionManager/butterfly at 0x1f4b950>

Let me know if you need more information.