Normally I run standalone with no NFS mounts, so I've never run across this
before.  A fellow NetBSD/SPARC 1.0 user at JPL asked me about a problem he
has with NFS mounting a SunOS 4.1.3 Sun onto his NetBSD 1.0 SPARCstation.
So I tried it myself, and sure enough ran into something similar:

In the following, "netbsd4me" is the NetBSD/SPARC 1.0 system, and "mipl7" is
a SPARCstation 2 running SunOS 4.1.3.

netbsd4me:1:27 % sudo mount -t nfs mipl7:/var/spool/mail /mnt
netbsd4me:1:28 % 

No complaints.

netbsd4me:1:28 % df
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/sd0a       16575     9428     5489    63%    /
...
/dev/sd3c     1014799   474311   489748    49%    /usr/othersrc
mfs:25          19727        4    18736     0%    /tmp
procfs              4        4        0   100%    /proc
AFS          72000000        0 72000000     0%    /afs

No mention of the NFS mount, however.  Did it work?

netbsd4me:1:29 % df -n
Filesystem            1K-blocks     Used    Avail Capacity  Mounted on
/dev/sd0a                 16575     9428     5489    63%    /
...
/dev/sd3c               1014799   474311   489748    49%    /usr/othersrc
mfs:25                    19727        4    18736     0%    /tmp
procfs                        4        4        0   100%    /proc
AFS                    72000000        0 72000000     0%    /afs
mipl7:/var/spool/mail         0        0        0   100%    /mnt
mipl7:/var/spool/mail         0        0        0   100%    /mnt

netbsd4me:1:30 % sudo mount -vt nfs mipl7:/var/spool/mail /mnt
mount: statfs /mnt: Permission denied

One would assume so, based on this.  Can't mount over the mount.

netbsd4me:1:31 % ls -ldg /mnt
ls: /mnt: Permission denied

Uh oh.

netbsd4me:1:32 % sudo ls -ldg /mnt
ls: /mnt: Permission denied

Uh oh again.

netbsd4me:1:33 % mount
/dev/sd0a on / type ufs (local)
...
AFS on /afs type afs
mipl7:/var/spool/mail on /mnt type nfs
mipl7:/var/spool/mail on /mnt type nfs

Well, "mount" thinks it's mounted ...

netbsd4me:1:34 % ls -ldg /mnt
ls: /mnt: Permission denied

netbsd4me:1:35 % ls -lg /
ls: mnt: Permission denied
total 2758
...

netbsd4me:1:35 % sudo umount -vat nfs
netbsd4me:1:36 % 

Doesn't seem to think anything's mounted, yet I can't access the mount point
in any way anymore.

I don't think the permissions on either the remote system's /var/spool/mail
or the local NetBSD system's underlying /mnt were strange/weird in any way.
Looks like I'm going to have to reboot to clear this condition.

To quote a Greywolfism, Am I Missing Something?

	- Greg