This is a trace I get in my shell periodically (once per minute approximately), but not in the logs.

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/eventlet/hubs/hub.py", line 346, in fire_timers
    timer()
  File "/usr/lib/python2.7/dist-packages/eventlet/hubs/timer.py", line 56, in __call__
    cb(*args, **kw)
  File "/usr/lib/python2.7/dist-packages/eventlet/greenthread.py", line 194, in main
    result = function(*args, **kwargs)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/service.py", line 557, in run_alarm_action
    stk = parser.Stack.load(admin_context, stack=stack)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/parser.py", line 139, in load
    stack.timeout, resolve_data, stack.disable_rollback)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/parser.py", line 99, in __init__
    for (name, data) in template_resources.items())
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/parser.py", line 99, in <genexpr>
    for (name, data) in template_resources.items())
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/resource.py", line 153, in __new__
    return ResourceClass(name, json, stack)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/resource.py", line 163, in __init__
    self.t = stack.resolve_static_data(json_snippet)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/parser.py", line 510, in resolve_static_data
    return resolve_static_data(self.t, self, self.parameters, snippet)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/parser.py", line 535, in resolve_static_data
    template.reduce_joins])
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/parser.py", line 557, in transform
    data = t(data)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 120, in resolve_availability_zones
    return _resolve(match_get_az, handle_get_az, s)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 381, in _resolve
    return dict((k, recurse(v)) for k, v in snippet.items())
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 381, in <genexpr>
    return dict((k, recurse(v)) for k, v in snippet.items())
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 374, in <lambda>
    recurse = lambda s: _resolve(match, handle, s)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 381, in _resolve
    return dict((k, recurse(v)) for k, v in snippet.items())
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 381, in <genexpr>
    return dict((k, recurse(v)) for k, v in snippet.items())
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 374, in <lambda>
    recurse = lambda s: _resolve(match, handle, s)
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 380, in _resolve
    return handle(recurse(v))
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/template.py", line 118, in handle_get_az
    return stack.get_availability_zones()
  File "/usr/local/lib/python2.7/dist-packages/heat/engine/parser.py", line 506, in get_availability_zones
    self.clients.nova().availability_zones.list(detailed=False)]
AttributeError: 'NoneType' object has no attribute 'availability_zones'