On Illumos, ZFS will validate that the "sharenfs" parameter is valid. It will refuse to set it to an invalid value. But FreeBSD does not validate it. This could cause mountd to crash or misbehave.
Note that this is _not_ a security vulnerability, even in combination with ZFS delegations, because /etc/zfs/exports may only be updated by root, even when the sharenfs property is delegated.
$ sudo zpool create foo da0
$ sudo zfs set sharenfs=$'XXX\n/usr/home' foo
$ sudo cat /etc/zfs/exports
# !!! DO NOT EDIT THIS FILE MANUALLY !!!