This could, very occasionally, cause:

```
tests.test_visibility.FilterEventsForServerTestCase.test_large_room
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/src/tests/rest/media/v1/test_media_storage.py", line 86, in test_ensure_media_is_in_local_cache
    self.wait_on_thread(x)
  File "/src/tests/unittest.py", line 296, in wait_on_thread
    self.reactor.advance(0.01)
  File "/src/.tox/py35/lib/python3.5/site-packages/twisted/internet/task.py", line 826, in advance
    self._sortCalls()
  File "/src/.tox/py35/lib/python3.5/site-packages/twisted/internet/task.py", line 787, in _sortCalls
    self.calls.sort(key=lambda a: a.getTime())
builtins.ValueError: list modified during sort

tests.rest.media.v1.test_media_storage.MediaStorageTests.test_ensure_media_is_in_local_cache
```