That fails consistently:
Python 2.6.2 (r262:71600, Jun  4 2009, 16:07:26) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> r,w=os.pipe()
>>> os.lseek(r,0,1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 29] Illegal seek
>>>
Python 3.0.1 (r301:69556, Jun  4 2009, 16:07:22) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> r,w=os.pipe()
>>> os.lseek(r,0,1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 29] Illegal seek
>>>
Python 3.1rc2 (r31rc2:73411, Jun 15 2009, 10:56:49) [C] on aix5
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> r,w=os.pipe()
>>> os.lseek(r,0,1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 29] Illegal seek
>>>