1 files changed, 3 insertions, 0 deletions
diff --git a/synapse/util/caches/stream_change_cache.py b/synapse/util/caches/stream_change_cache.py
index 235f64049c..c61d36a82e 100644
--- a/synapse/util/caches/stream_change_cache.py
+++ b/synapse/util/caches/stream_change_cache.py
@@ -126,6 +126,9 @@ class StreamChangeCache(object):
         """
         assert type(stream_pos) is int
 
+        # FIXME: add a sanity check here that we are not overwriting existing
+        # data in self._cache
+
         if stream_pos > self._earliest_known_stream_pos:
             old_pos = self._entity_to_key.get(entity, None)
             if old_pos is not None: