Running the attached files produces:
ONE PASS

However if you uncomment line 46 and run the file again, you don't get any output.

A pdb session of the file produces:
[mtebeka@lakshmi:python-bug - 14:14] $pdb yield.py 
> /home/mtebeka/work/python-bug/yield.py(3)<module>()
-> from imaplib import IMAP4
(Pdb) b 72
Breakpoint 1 at /home/mtebeka/work/python-bug/yield.py:72
(Pdb) c
> /home/mtebeka/work/python-bug/yield.py(72)main()
-> one_pass()
(Pdb) s
--Call--
/usr/local/lib/python2.5/bdb.py:318: RuntimeWarning: tp_compare didn't return -1 or -2 for exception
  i = max(0, len(stack) - 1)
The program finished and will be restarted
> /home/mtebeka/work/python-bug/yield.py(3)<module>()
-> from imaplib import IMAP4
(Pdb) q
[mtebeka@lakshmi:python-bug - 14:15] $

(Sorry for the length of the example, this is the smaller version that still reproduces the bug)