diff --git a/Doc/library/functools.rst b/Doc/library/functools.rst
index 97136b234084fc..221c0712c7c96a 100644
--- a/Doc/library/functools.rst
+++ b/Doc/library/functools.rst
@@ -57,6 +57,10 @@ The :mod:`functools` module defines the following functions:
    another thread makes an additional call before the initial call has been
    completed and cached.
 
+   Call-once behavior is not guaranteed because locks are not held during the
+   function call. Potentially another call with the same arguments could
+   occur while the first call is still running.
+
    .. versionadded:: 3.9