diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -445,11 +445,17 @@ class ui(object):
             if default is _unset:
                 default = None
         else:
+            item = self._knownconfig.get(section, {}).get(name)
             if default is _unset:
                 default = None
-                item = self._knownconfig.get(section, {}).get(name)
                 if item is not None:
                     default = item.default
+            elif item is not None:
+                msg = ("specifying a default value for a registered "
+                       "config item: '%s.%s' '%s'")
+                msg %= (section, name, default)
+                self.develwarn(msg, 1, 'warn-config-default')
+
             alternates = [name]
 
         for n in alternates:
diff --git a/tests/test-devel-warnings.t b/tests/test-devel-warnings.t
--- a/tests/test-devel-warnings.t
+++ b/tests/test-devel-warnings.t
@@ -193,4 +193,23 @@ Old style deprecation warning
 
   $ HGEMITWARNINGS= hg nouiwarning
 
+Test warning on config option access and registration
+
+  $ cat << EOF > ${TESTTMP}/buggyconfig.py
+  > """A small extension that tests our developer warnings for config"""
+  > 
+  > from mercurial import registrar
+  > 
+  > cmdtable = {}
+  > command = registrar.command(cmdtable)
+  > 
+  > @command('buggyconfig')
+  > def cmdbuggyconfig(ui, repo):
+  >     repo.ui.config('ui', 'quiet', False)
+  >     repo.ui.config('ui', 'interactive', None)
+  > EOF
+
+  $ hg --config "extensions.buggyconfig=${TESTTMP}/buggyconfig.py" buggyconfig
+  devel-warn: specifying a default value for a registered config item: 'ui.quiet' 'False' at: $TESTTMP/buggyconfig.py:* (cmdbuggyconfig) (glob)
+
   $ cd ..