diff --git a/synapse/storage/__init__.py b/synapse/storage/__init__.py
index 30ce3789005aa20172bb8519e84b22b1db80a894..a63c59a8a25d8893809165f0f78e7318bbbac721 100644
--- a/synapse/storage/__init__.py
+++ b/synapse/storage/__init__.py
@@ -239,6 +239,12 @@ class DataStore(RoomMemberStore, RoomStore,
             event.internal_metadata.get_dict()
         )
 
+        # If we have already persisted this event, we don't need to do any
+        # more processing.
+        # The processing above must be done on every call to persist event,
+        # since they might not have happened on previous calls. For example,
+        # if we are persisting an event that we had persisted as an outlier,
+        # but is no longer one.
         if have_persisted:
             if not outlier:
                 sql = (