diff --git a/HACKING.rst b/HACKING.rst
index a0595952c..b12291a1c 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -12,6 +12,7 @@ Neutron Specific Commandments
 - [N321] Validate that jsonutils module is used instead of json
 - [N322] We do not use @authors tags in source files. We have git to track
   authorship.
+- [N323] assert_called_once() is not a valid method
 
 Creating Unit Tests
 -------------------