Are there limits to what you can do inside asynchronous URL fetch callbacks?

Question

I'm experimenting with the asynchronous URL fetch interface. But when callbacks are used on the production environment, I get an AssertionError.

Is this caused by datastore access in the callback function? What limits are there in the callback function? Are there other APIs that cannot run asynchronously?

Here's the stack...

('The Wait() request was interrupted by an exception from another callback:', AssertionError('1',))
Traceback (most recent call last):
  File "/base/python_runtime/python_lib/versions/1/google/appengine/ext/webapp/__init__.py", line 513, in __call__
    handler.post(*groups)
  File "/base/data/home/apps/smsmybus/15.344643308619037909/main.py", line 153, in post
    self.get()
  File "/base/data/home/apps/smsmybus/15.344643308619037909/main.py", line 182, in get
    bus.aggregateBusesAsynch(smsBody,self.request.get('SmsSid'),self.request.get('From'))
  File "/base/data/home/apps/smsmybus/15.344643308619037909/bus.py", line 296, in aggregateBusesAsynch
    rpc.wait()
  File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_stub_map.py", line 460, in wait
    self.__rpc.Wait()
  File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_rpc.py", line 112, in Wait
    rpc_completed = self._WaitImpl()
  File "/base/python_runtime/python_lib/versions/1/google/appengine/runtime/apiproxy.py", line 108, in _WaitImpl
    rpc_completed = _apphosting_runtime___python__apiproxy.Wait(self)
  File "/base/python_runtime/python_lib/versions/1/google/appengine/runtime/apiproxy.py", line 171, in __MakeCallDone
    self.__Callback()
  File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_rpc.py", line 161, in __Callback
    self.callback()
  File "/base/data/home/apps/smsmybus/15.344643308619037909/bus.py", line 265, in <lambda>
    return lambda: handle_result(rpc,stopID,routeID,sid,directionID)
  File "/base/data/home/apps/smsmybus/15.344643308619037909/bus.py", line 252, in handle_result
    stop.put()
  File "/base/python_runtime/python_lib/versions/1/google/appengine/ext/db/__init__.py", line 893, in put
    return datastore.Put(self._entity, rpc=rpc)
  File "/base/python_runtime/python_lib/versions/1/google/appengine/api/datastore.py", line 291, in Put
    'datastore_v3', 'Put', req, datastore_pb.PutResponse(), rpc)
  File "/base/python_runtime/python_lib/versions/1/google/appengine/api/datastore.py", line 194, in _MakeSyncCall
    rpc.wait()
  File "/base/python_runtime/python_lib/versions/1/google/appengine/api/apiproxy_stub_map.py", line 461, in wait
    assert self.__rpc.state == apiproxy_rpc.RPC.FINISHING, repr(self.state)
InterruptedError: ('The Wait() request was interrupted by an exception from another callback:', AssertionError('1',))