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The documentation for flush says

"Flushes changes made to the in-memory copy of a file back
to disk."

But it doesn't do that, and we all agree it shouldn't do
that. So I would claim that it is an error to use .flush on
an mmap object that was opened in ACCESS_COPY. 

This is like trying to write to a file that was opened for
reading only: one *could* declare that the write just does
nothing, but it helps the developer more if you get an
exception, because the code is likely wrong (i.e. not
following the likely intentions of the author).