> >>which can happen if e.g. the directory was renamed, or moved.
> >
> >probably it is the point.  Server was rebooted because of system update,
> >and my procedure for system updating includes executing some scripts.
> >Directory /stuffy was removed and then recreated again.
> 
> removed and recreated means we're talking about a different directory, 
> although with the same name. But they have different inode numbers.
> 
> >So, the reason why /flam and /flam/volatile remain mounted and
> >/flam/stuffy doesn't is clear.
> 
> Really? If you remove flam:/stuffy, and recreate it, nfs will not be able 
> to retain that mount. You're basically very lost. You'll have do umount, 
> and mount it again. Doing a mount -u is not the same thing.

Do not understand your "Really?".  I wrote that I recalled
that directory /stuffy was removed and recreated.  And I wrote about
reason why flam:/volatile remains mounted and flam:/stuffy
appears lost.  And I wrote this reason is clear.

> >But anyway it seems not very beatiful that client can not umount
> >this filesystem.
> 
> You shouldn't have any problems doing an umount. Did you have /flam/stuffy 
> in your fstab? Otherwise I'm not sure you can do the umount the way you're 
> trying. Perhaps try "umount flam:/stuffy"

However the problem is that I really cannot umount it.
It is in fstab, but actually it does not matter because
it was always possible to umount either by device or by mount_point.

# mount
/dev/wd0a on / type ffs (noatime, nodevmtime, NFS exported, local)
/dev/cgd1e on /volatile_sekuritat type ffs (noatime, soft dependencies, local)
/dev/cgd1f on /sekuritat type ffs (noatime, soft dependencies, local)
procfs on /proc type procfs (read-only, local)
kernfs on /kern type kernfs (read-only, local)
mfs:341 on /tmp type mfs (synchronous, local)
/dev/cgd3f on /volatile type ffs (noatime, soft dependencies, local)
/dev/cgd3g on /usr/local type ffs (nodev, noatime, local)
/dev/cgd3e on /home type ffs (nosuid, nodev, noatime, soft dependencies, local)
/dev/cgd3h on /stuffz type ffs (read-only, noexec, nosuid, nodev, noatime, local)
flam:/ on /flam type nfs (read-only)
flam:/volatile on /flam/volatile type nfs
flam:/stuffy on /flam/stuffy type nfs (read-only, nosuid, nodev)

# umount flam:/stuffy
umount: /flam/stuffy: Invalid argument

I just cannot umount it.  And if I mount it again, then
this filesystem appears in df twice.

So this is a bug, probably I should send-pr it..