I've looked at the logs.  Here's the relevant bits from maas-django.log and maas.log:

maas-django.log:
ERROR 2014-10-28 15:40:35,954 maasserver ################################ Exception: An error occurred in the current transaction. You can't execute queries until the end of the 'atomic' block. ################################
ERROR 2014-10-28 15:40:35,979 maasserver Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/django/core/handlers/base.py", line 112, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python2.7/dist-packages/django/views/decorators/vary.py", line 19, in inner_func
    response = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/piston/resource.py", line 167, in __call__
    result = self.error_handler(e, request, meth, em_format)
  File "/usr/lib/python2.7/dist-packages/piston/resource.py", line 165, in __call__
    result = meth(request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/support.py", line 189, in dispatch
    return function(self, request, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/api/nodes.py", line 312, in start
    [system_id], request.user, user_data=user_data)
  File "/usr/lib/python2.7/dist-packages/maasserver/models/node.py", line 416, in start_nodes
    claims = node.claim_static_ip_addresses()
  File "/usr/lib/python2.7/dist-packages/maasserver/models/node.py", line 1506, in claim_static_ip_addresses
    static_ips = mac.claim_static_ips()
  File "/usr/lib/python2.7/dist-packages/maasserver/models/macaddress.py", line 302, in claim_static_ips
    interface, alloc_type, requested_address)
  File "/usr/lib/python2.7/dist-packages/maasserver/models/macaddress.py", line 222, in _allocate_static_address
    alloc_type, requested_address=requested_address)
  File "/usr/lib/python2.7/dist-packages/maasserver/models/staticipaddress.py", line 169, in allocate_new
    requested_address, alloc_type, user)
  File "/usr/lib/python2.7/dist-packages/maasserver/models/staticipaddress.py", line 103, in _attempt_allocation
    ipaddress.save()
  File "/usr/lib/python2.7/dist-packages/maasserver/models/cleansave.py", line 38, in save
    return super(CleanSave, self).save(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/maasserver/models/timestampedmodel.py", line 51, in save
    current_time = now()
  File "/usr/lib/python2.7/dist-packages/maasserver/models/timestampedmodel.py", line 31, in now
    cursor.execute("select now()")
  File "/usr/lib/python2.7/dist-packages/django/db/backends/util.py", line 47, in execute
    self.db.validate_no_broken_transaction()
  File "/usr/lib/python2.7/dist-packages/django/db/backends/__init__.py", line 365, in validate_no_broken_transaction
    "An error occurred in the current transaction. You can't "
TransactionManagementError: An error occurred in the current transaction. You can't execute queries until the end of the 'atomic' block.

maas.log:
Oct 28 15:40:32 maas-trusty-back-may22 maas.node: [INFO] hayward-42 allocated to user oil-slave-3
Oct 28 15:40:35 maas-trusty-back-may22 maas.node: [INFO] anahuac.oil: Starting monitor: {u'deadline': datetime.datetime(2014, 10, 28, 16, 20, 35, 952862, tzinfo=<twisted.protocols.amp._FixedOffsetTZInfo object at 0x7fc3e64e5810>), u'id': u'node-f626062e-ae16-11e3-b194-00163efc5068', u'context': {u'node_status': 9, u'timeout': 2400.0}}
Oct 28 15:40:36 maas-trusty-back-may22 maas.dns: [INFO] Generating new DNS zone file for oil

So it looks like it was trying to set up a deployment monitor at the time of that error in the appserver log.