I was trying to destroy a filesystem and I was baffled by the following
zfs destroy -r rpool/test/opt
cannot destroy 'rpool/test/opt/csw@20111101_1405': dataset already exists
zfs destroy -r rpool/test/opt/csw@20111101_1405
cannot destroy 'rpool/test/opt/csw@20111101_1405': snapshot is cloned
It turns out there was a zfs receive writing to the filesystem.
A more sensible error would have been "dataset is busy".
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