diff -urNp Python-2.7.5.orig/Python/bltinmodule.c Python-2.7.5/Python/bltinmodule.c
--- Python-2.7.5.orig/Python/bltinmodule.c	2013-05-12 06:32:53.000000000 +0300
+++ Python-2.7.5/Python/bltinmodule.c	2013-07-12 21:19:11.000000000 +0300
@@ -2419,7 +2419,20 @@ builtin_sum(PyObject *self, PyObject *ar
              empty = []
              sum([[x] for x in range(10)], empty)
 
-           would change the value of empty. */
+           would change the value of empty.
+
+           And what is more important it would break the code, that
+           has __iadd__ different from __add__ e.g. due to coercion:
+
+           >>> from numpy import array
+           >>> a = array([1, 2, 3], dtype=int)
+           >>> b = array([1.4, 2.5, 3.6], dtype=float)
+           >>> a + b
+           array([ 2.4,  4.5,  6.6])
+           >>> a += b
+           >>> a
+           array([2, 4, 6])
+        */
         temp = PyNumber_Add(result, item);
         Py_DECREF(result);
         Py_DECREF(item);