Why list(six.iteritems(d)) is bad.

gistfile1.txt

>>> d = { "a": "b" }
>>> i = iter(d.iteritems())
>>> i
<dictionary-itemiterator object at 0x1072d8cb0>
>>> d["c"] = "d"
>>> list(i)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration