Leaked mountpoints

I'm seeing a lot of mountpoints "leaked", i.e. still being listed in
the mount table but not actually being mounted (or unmountable).

haessal# mount -v
/dev/da0s1a on / (ufs, NFS exported, local, fsid 09e918411e72170c)
devfs on /dev (devfs, local, fsid 00ff000101000000)
/dev/da0s1e on /c (ufs, NFS exported, local, fsid 09e9184151f3b848)
/dev/da0s1d on /usr (ufs, NFS exported, local, soft-updates, fsid 0ae918413c7f72af)
devfs on /var/named/dev (devfs, local, fsid 01ff000101000000)

^-- these are all legitimately mounted, but these --v are leaked.

/c/portbuild/i386/6/ports on /c/pkgbuild/6/chroot/40248/a/ports (nullfs, local, read-only, fsid 61ff000202000000)
/c/portbuild/i386/6/src on /c/pkgbuild/6/chroot/40248/usr/src (nullfs, local, read-only, fsid 62ff000202000000)
devfs on /c/pkgbuild/6/chroot/40248/dev (devfs, local, fsid 63ff000101000000)
linprocfs on /c/pkgbuild/6/chroot/40248/compat/linux/proc (linprocfs, local, fsid 65ff000808000000)
/c/portbuild/i386/6/ports on /c/pkgbuild/6/chroot/56024/a/ports (nullfs, local, read-only, fsid 6cff000202000000)
/c/portbuild/i386/6/src on /c/pkgbuild/6/chroot/56024/usr/src (nullfs, local, read-only, fsid 6dff000202000000)
devfs on /c/pkgbuild/6/chroot/56024/dev (devfs, local, fsid 6eff000101000000)
linprocfs on /c/pkgbuild/6/chroot/56024/compat/linux/proc (linprocfs, local, fsid 6fff000808000000)
/c/portbuild/i386/6/ports on /c/pkgbuild/6/chroot/62793/a/ports (nullfs, local, read-only, fsid 71ff000202000000)
/c/portbuild/i386/6/src on /c/pkgbuild/6/chroot/62793/usr/src (nullfs, local, read-only, fsid 72ff000202000000)
devfs on /c/pkgbuild/6/chroot/62793/dev (devfs, local, fsid 73ff000101000000)

haessal# umount -f 73ff000101000000
umount: unmount of /c/pkgbuild/6/chroot/62793/dev failed: No such file or directory
umount: retrying using path instead of file system ID
umount: unmount of /c/pkgbuild/6/chroot/62793/dev failed: No such file or directory

haessal# mount -v
/dev/da0s1a on / (ufs, NFS exported, local, fsid 09e918411e72170c)
devfs on /dev (devfs, local, fsid 00ff000101000000)
/dev/da0s1e on /c (ufs, NFS exported, local, fsid 09e9184151f3b848)
/dev/da0s1d on /usr (ufs, NFS exported, local, soft-updates, fsid 0ae918413c7f72af)
devfs on /var/named/dev (devfs, local, fsid 01ff000101000000)
/c/portbuild/i386/6/ports on /c/pkgbuild/6/chroot/40248/a/ports (nullfs, local, read-only, fsid 61ff000202000000)
/c/portbuild/i386/6/src on /c/pkgbuild/6/chroot/40248/usr/src (nullfs, local, read-only, fsid 62ff000202000000)
devfs on /c/pkgbuild/6/chroot/40248/dev (devfs, local, fsid 63ff000101000000)
linprocfs on /c/pkgbuild/6/chroot/40248/compat/linux/proc (linprocfs, local, fsid 65ff000808000000)
/c/portbuild/i386/6/ports on /c/pkgbuild/6/chroot/56024/a/ports (nullfs, local, read-only, fsid 6cff000202000000)
/c/portbuild/i386/6/src on /c/pkgbuild/6/chroot/56024/usr/src (nullfs, local, read-only, fsid 6dff000202000000)
devfs on /c/pkgbuild/6/chroot/56024/dev (devfs, local, fsid 6eff000101000000)
linprocfs on /c/pkgbuild/6/chroot/56024/compat/linux/proc (linprocfs, local, fsid 6fff000808000000)
/c/portbuild/i386/6/ports on /c/pkgbuild/6/chroot/62793/a/ports (nullfs, local, read-only, fsid 71ff000202000000)
/c/portbuild/i386/6/src on /c/pkgbuild/6/chroot/62793/usr/src (nullfs, local, read-only, fsid 72ff000202000000)
devfs on /c/pkgbuild/6/chroot/62793/dev (devfs, local, fsid 73ff000101000000)

haessal# ls /c/pkgbuild/6/chroot/62793/dev
ls: /c/pkgbuild/6/chroot/62793/dev: No such file or directory

haessal# ls /c/pkgbuild/6/chroot/62793/
ls: /c/pkgbuild/6/chroot/62793/: No such file or directory