foo.diff

diff --git a/docs/ref/models/relations.txt b/docs/ref/models/relations.txt
index eb5c815..1faca0b 100644
--- a/docs/ref/models/relations.txt
+++ b/docs/ref/models/relations.txt
@@ -150,6 +150,10 @@ Related objects reference
         If ``clear=True``, the ``clear()`` method is called instead and the
         whole set is added at once.
 
+        Note that ``set()`` being a compound operation it is subjected to race
+        conditions. For instance new objects may be added to the database in
+        between the call to ``clear()`` and the call to ``add()``.
+
     .. note::
 
        Note that ``add()``, ``create()``, ``remove()``, ``clear()``, and