Index: docs/db-api.txt
===================================================================
--- docs/db-api.txt	(revision 6048)
+++ docs/db-api.txt	(working copy)
@@ -799,6 +799,9 @@
 
         Entry.objects.extra(where=['headline=%s'], params=['Lennon'])
 
+    The combined number of placeholders in the list of strings for ``select`` or
+    ``where`` should equal the number of values in the ``params`` list.
+
 QuerySet methods that do not return QuerySets
 ---------------------------------------------