diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -1079,6 +1079,7 @@ class TestResult(unittest._TextTestResul
         else:
             if not self._options.nodiff:
                 self.stream.write('\nERROR: %s output changed\n' % test)
+            self.stream.write('!')
 
     def addError(self, *args, **kwargs):
         super(TestResult, self).addError(*args, **kwargs)