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"Does it tell you more than _just_ that?  It doesn't for me."

All requests against the file in question were issued by the
python process; there's no sign of virus checkers or other
external applications.

Also, whenever things failed, there were always multiple
requests for cmd.exe (caused by os.system) between the WRITE
request and the failing OPEN request.
 
My feel, after staring at filemon output, is that this is a
problem in the Windows file I/O layer.  NTFS queues the
various operations, and calling an external process with
stuff still in the queue messes up the request scheduling.