ZFS allows datasets named .. or .
ZFS allows datasets to be named either '..' or '.'
These names when the default mountpoint is used will make ZFS very unhappy.
In the process of doing zfs create tank/.system if your cat jumps up on your keyboard while you have zfs create tank/. typed out and hits enter ZFS will helpfully create a dataset named tank/. for you and mount it to tank/. or even worse if you are mounting root off tank /.
Hilarity will then ensue...how much hilarity depends on whether you mount root off tank or not.