diff --git a/hgext3rd/configwarn.py b/hgext3rd/configwarn.py
new file mode 100644
--- /dev/null
+++ b/hgext3rd/configwarn.py
@@ -0,0 +1,53 @@
+# configwarn.py - warn unsupported user configs
+#
+# Copyright 2017 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+"""warn unsupported user configs
+
+Config::
+
+    [configwarn]
+    # Config names that are supposed to be set by system config and not
+    # overrided by user config.
+    systemconfigs = diff.git, extensions.hggit
+"""
+
+from __future__ import absolute_import
+
+from mercurial.i18n import _
+from mercurial import (
+    rcutil,
+    registrar,
+)
+
+configtable = {}
+configitem = registrar.configitem(configtable)
+
+configitem('configwarn', 'systemconfigs', default=[])
+
+def reposetup(ui, repo):
+    # use reposetup, not uisetup to work better with chg and it checks reporc.
+    if not repo.local():
+        return
+
+    nonsystempaths = set(rcutil.userrcpath() + [repo.vfs.join('hgrc')])
+    systemconfigs = ui.configlist('configwarn', 'systemconfigs')
+
+    for configname in systemconfigs:
+        if '.' not in configname:
+            continue
+
+        section, name = configname.split('.', 1)
+        source = ui.configsource(section, name)
+
+        if ':' not in source:
+            continue
+
+        path, lineno = source.split(':', 1)
+        if path in nonsystempaths and lineno.isdigit():
+            ui.warn(_('warning: overriding config %s is unsupported (hint: '
+                      'remove line %s from %s to resolve this issue)\n')
+                    % (configname, lineno, path))
diff --git a/tests/test-configwarn.t b/tests/test-configwarn.t
new file mode 100644
--- /dev/null
+++ b/tests/test-configwarn.t
@@ -0,0 +1,54 @@
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > configwarn=$TESTDIR/../hgext3rd/configwarn.py
+  > [configwarn]
+  > systemconfigs=diff.git, phases.publish
+  > [alias]
+  > noop=log -r null -T '{files}'
+  > EOF
+
+Need to override rcutil.userrcpath to test properly without side-effects
+
+  $ cat >> $TESTTMP/rcpath.py <<EOF
+  > from __future__ import absolute_import
+  > from mercurial import encoding, rcutil
+  > def userrcpath():
+  >     return [encoding.environ[b'USERHGRC']]
+  > rcutil.userrcpath = userrcpath
+  > EOF
+
+  $ cat >> $HGRCPATH <<EOF
+  > [extensions]
+  > rcpath=$TESTTMP/rcpath.py
+  > EOF
+
+  $ USERHGRC="$TESTTMP/userhgrc"
+  $ HGRCPATH="$HGRCPATH:$USERHGRC"
+  $ export USERHGRC
+
+Config set by system config files or command line flags are fine
+
+  $ hg init
+  $ hg noop
+  $ hg noop --config diff.git=1
+
+Config set by user config will generate warnings
+
+  $ cat >> $USERHGRC <<EOF
+  > [diff]
+  > git=0
+  > EOF
+
+  $ hg noop
+  warning: overriding config diff.git is unsupported (hint: remove line 2 from $TESTTMP/userhgrc to resolve this issue)
+
+Config set by repo will generate warnings
+
+  $ cat >> .hg/hgrc << EOF
+  > [phases]
+  > publish=1
+  > EOF
+
+  $ hg noop
+  warning: overriding config diff.git is unsupported (hint: remove line 2 from $TESTTMP/userhgrc to resolve this issue)
+  warning: overriding config phases.publish is unsupported (hint: remove line 2 from $TESTTMP/.hg/hgrc to resolve this issue)