Confirmed on OSX 64bits with Mark's sample.

$ python3.3
Python 3.3.0 (default, Jan 24 2013, 08:28:09) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> dir1 = "D:\\Bug reports\\Python\\test\\sub-fcc"
>>> dir1 += "\\bulk"
>>> s1 = ascii(dir1.encode('unicode_internal'))
>>> dir1 += "\\bulk"
>>> s1 = ascii(dir1.encode('unicode_internal'))
>>> len(s1), s1[499:]
(586, "00k\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\xf0\\xbda\\x00\\x01\\x00\\x00\\x00X\\x1da\\x00\\x01\\x00\\x00\\x00'")
>>> dir1 = "D:\\Bug reports\\Python\\test\\sub-fcc"
>>> dir1 += "\\bulk"
>>> s1 = ascii(dir1.encode('unicode_internal'))
>>> dir1 += "\\bulk"
>>> s1 = ascii(dir1.encode('unicode_internal'))
>>> len(s1), s1[499:]
(586, "00k\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x10\\xbca\\x00\\x01\\x00\\x00\\x00X\\x16a\\x00\\x01\\x00\\x00\\x00'")
>>> dir1 = "D:\\Bug reports\\Python\\test\\sub-fcc"
>>> dir1 += "\\bulk"
>>> s1 = ascii(dir1.encode('unicode_internal'))
>>> dir1 += "\\bulk"
>>> s1 = ascii(dir1.encode('unicode_internal'))
>>> len(s1), s1[499:]
(595, "00k\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00'")
>>> dir1 = "D:\\Bug reports\\Python\\test\\sub-fcc"
>>> dir1 += "\\bulk"
>>> s1 = ascii(dir1.encode('unicode_internal'))
>>> dir1 += "\\bulk"
>>> s1 = ascii(dir1.encode('unicode_internal'))
>>> len(s1), s1[499:]
(586, "00k\\x00\\x00\\x00\\x00\\x00\\x00\\x00p\\xbba\\x00\\x01\\x00\\x00\\x00\\x88\\x14a\\x00\\x01\\x00\\x00\\x00'")
>>> 


Darwin Kernel Version 10.8.0: Tue Jun  7 16:32:41 PDT 2011; root:xnu-1504.15.3~1/RELEASE_X86_64 x86_64