I thought about this too, but I don't know of a way to provoke an error.
(Other than passing in illegal values, but the code tries to catch
those. And btw, raises an Exception on windows :)

One could currently pass in a negative value for size (this isn't caught
in the checks). e.g.

m.flush(500, -500) gives an 'error: [Errno 22] Invalid argument' on linux.

but then you don't want to rely on another bug for testing purposes.