That does the TombstonedTaskError mean? It is being raised while trying to add a task to the queue, from a cron-job:

Traceback (most recent call last):
  File "/base/python_lib/versions/1/google/appengine/ext/webapp/__init__.py", line 501, in __call__
    handler.get(*groups)
  File "/base/data/home/apps/.../tasks.py", line 132, in get
    ).add(queue_name = 'userfeedcheck')
  File "/base/python_lib/versions/1/google/appengine/api/labs/taskqueue/taskqueue.py", line 495, in add
    return Queue(queue_name).add(self)
  File "/base/python_lib/versions/1/google/appengine/api/labs/taskqueue/taskqueue.py", line 563, in add
    self.__TranslateError(e)
  File "/base/python_lib/versions/1/google/appengine/api/labs/taskqueue/taskqueue.py", line 619, in __TranslateError
    raise TombstonedTaskError(error.error_detail)
TombstonedTaskError

Searching the documentation only has the following to say:

exception TombstonedTaskError(InvalidTaskError)
    Task has been tombstoned.

..which isn't particularly helpful.

I couldn't find anything useful in the App Engine code either..