diff --git a/tests/data/stubs/stub-su.py b/tests/data/stubs/stub-su.py
index c32c91de..6782490c 100755
--- a/tests/data/stubs/stub-su.py
+++ b/tests/data/stubs/stub-su.py
@@ -4,6 +4,16 @@ import json
 import os
 import subprocess
 import sys
+import time
+
+# #363: old input loop would fail to spot auth failure because of scheduling
+# vs. su calling write() twice.
+if 'DO_SLOW_AUTH_FAILURE' in os.environ:
+    os.write(2, 'su: ')
+    time.sleep(0.5)
+    os.write(2, 'incorrect password\n')
+    os._exit(1)
+
 
 os.environ['ORIGINAL_ARGV'] = json.dumps(sys.argv)
 os.environ['THIS_IS_STUB_SU'] = '1'
diff --git a/tests/su_test.py b/tests/su_test.py
index 760dd50d..447b00fc 100644
--- a/tests/su_test.py
+++ b/tests/su_test.py
@@ -11,11 +11,11 @@ import testlib
 
 
 class ConstructorTest(testlib.RouterMixin, testlib.TestCase):
-    su_path = testlib.data_path('stubs/stub-su.py')
+    stub_su_path = testlib.data_path('stubs/stub-su.py')
 
     def run_su(self, **kwargs):
         context = self.router.su(
-            su_path=self.su_path,
+            su_path=self.stub_su_path,
             **kwargs
         )
         argv = eval(context.call(os.getenv, 'ORIGINAL_ARGV'))
@@ -28,6 +28,19 @@ class ConstructorTest(testlib.RouterMixin, testlib.TestCase):
 
 
 class SuTest(testlib.DockerMixin, testlib.TestCase):
+    stub_su_path = testlib.data_path('stubs/stub-su.py')
+
+    def test_slow_auth_failure(self):
+        # #363: old input loop would fail to spot auth failure because of
+        # scheduling vs. su calling write() twice.
+        os.environ['DO_SLOW_AUTH_FAILURE'] = '1'
+        try:
+            self.assertRaises(mitogen.su.PasswordError,
+                lambda: self.router.su(su_path=self.stub_su_path)
+            )
+        finally:
+            del os.environ['DO_SLOW_AUTH_FAILURE']
+
     def test_password_required(self):
         ssh = self.docker_ssh(
             username='mitogen__has_sudo',