If you set a mountpoint on a currently unmounted filesystem with one or more unmounted children, it appears that the children get mounted first, preventing the parent from mounting.
test case:
# zfs create z/space/test <- test placeholder
# zfs set mountpoint=none z/space/test <- unmount
# zfs create z/space/test/one <- create child
# zfs set mountpoint=/zz z/space/test <- mount
cannot mount '/zz': directory is not empty
# df -F zfs
Filesystem 1024-blocks Used Available Capacity Mounted on
z/space 45674496 1421018 42937575 4% /space
z/space/sommerfeld 45674496 1313592 42937575 3% /space/sommerfeld
z/space/test/one 45674496 18 42937575 1% /zz/one