diff -r 84c6b9384d6a -r 85a2db47ad50 mercurial/ui.py
--- a/mercurial/ui.py	Tue Oct 17 21:15:31 2017 +0200
+++ b/mercurial/ui.py	Mon Oct 16 17:41:27 2017 +0200
@@ -477,6 +477,10 @@
 
         if item is not None:
             alternates.extend(item.alias)
+        else:
+            msg = ("accessing unregistered config item: '%s.%s'")
+            msg %= (section, name)
+            self.develwarn(msg, 2, 'warn-config-unknown')
 
         if default is _unset:
             if item is None: