As far as I can tell, if a read was cancelled from a background thread,
the IO operation wasn't cancelled, so the system was poking at the
stack-allocated OVERLAPPED structure after it had been removed from the
stack, at which point something else might be occupying that region of
stack memory.
I'm pretty sure Address Sanitizer should be able to catch this sort of
thing so I'm surprised it didn't complain here.
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