diff --git a/synapse/util/async.py b/synapse/util/async.py
index 6f567bcaa6..5a1d545c96 100644
--- a/synapse/util/async.py
+++ b/synapse/util/async.py
@@ -38,6 +38,9 @@ class ObservableDeferred(object):
     deferred.
 
     If consumeErrors is true errors will be captured from the origin deferred.
+
+    Cancelling or otherwise resolving an observer will not affect the original
+    ObservableDeferred.
     """
 
     __slots__ = ["_deferred", "_observers", "_result"]