'countformat' does not appear to be handling the case where a format string is passed with no parenthesis or brackets correctly. Here is an
example of a usage that might cause issues from typedobject.c:

static PyObject*
slot_sq_slice(PyObject *self, Py_ssize_t i, Py_ssize_t j)
{
	static PyObject *getslice_str;

	if (PyErr_WarnPy3k("in 3.x, __getslice__ has been removed; "
			    "use __getitem__", 1) < 0)
		return NULL;
	return call_method(self, "__getslice__", &getslice_str,
		"nn", i, j);      <<<<<<< Maybe Bad Format Str <<<<<<<
}

The format string "nn" does not have any level markers so when it gets
processed by 'countformat' the count will be incremented 2 times by the
'default' case but when the end of the string is reached and the NULL character is processed bay "case '\0':".  The function will ignore the count variable and just return -1 anyway.  The error created is unmatched paren in format but 'level' is never checked to see if a paren
was even hit to begin with.

It might be that the case should be changed to look at level before assuming a error condition if strings are supposed to be processed as the one in the example above.  case '\0' should probably be doing something like:

         case '\0':
            if (level > 0) {   // Check If Level Was Incremented
                /* Premature end */
                PyErr_SetString(PyExc_SystemError,
                                "unmatched paren in format");
                return -1;
            }
            break;