diff -r ed8eefa6d57c Lib/test/test_exceptions.py
--- a/Lib/test/test_exceptions.py	Mon Mar 21 08:55:16 2011 +0100
+++ b/Lib/test/test_exceptions.py	Tue Mar 22 02:20:43 2011 +0100
@@ -708,6 +708,16 @@
             self.fail("RuntimeError not raised")
         self.assertEqual(wr(), None)
 
+    def test_new_returns_invalid_instance(self):
+        # See issue #11627.
+        class MyException(Exception):
+            def __new__(cls, *args):
+                return object()
+
+        def raise_():
+            raise MyException
+        self.assertRaises(TypeError, raise_)
+
 def test_main():
     run_unittest(ExceptionTests)
 
diff -r ed8eefa6d57c Python/ceval.c
--- a/Python/ceval.c	Mon Mar 21 08:55:16 2011 +0100
+++ b/Python/ceval.c	Tue Mar 22 02:20:43 2011 +0100
@@ -3249,6 +3249,13 @@
         value = PyObject_CallObject(exc, NULL);
         if (value == NULL)
             goto raise_error;
+        else if (!PyExceptionInstance_Check(value)) {
+            PyErr_Format(PyExc_TypeError,
+                         "calling %R should have returned an instance of "
+                         "BaseException, not %R",
+                         type, Py_TYPE(value));
+            goto raise_error;
+        }
     }
     else if (PyExceptionInstance_Check(exc)) {
         value = exc;