FS prob? df prob? or Infinite Improbability Drive?

[J.R. Oldroyd]

I've seen stats like this several times recently in the last few
months on different filesystems.

System is -current of 4/17 and has been up continuously for the 13
days since then.  World is also 4/17.  Everything was fine until
this morning, when:

# df /home
Filesystem  1K-blocks               Used             Avail Capacity  Mounted on
/dev/ad0s1a    253678             220128             13256    94%    /
devfs               1                  1                 0   100%    /dev
/dev/ad0s1d    253678                808            232576     0%    /tmp
/dev/ad0s1e    507630              74440            392580    16%    /var
/dev/ad0s1f    253678                 42            233342     0%    /var/mail
/dev/ad0s1g    253678                 74            233310     0%    /var/spool
/dev/ad2s1d   5077038            1356322           3314554    29%    /var/www
/dev/ad0s1h  32300804            7715462          22001278    26%    /usr
/dev/ad2s1e  26908468 -54043195503950556 54043195528706348 -218305257630%    /home
/dev/ad1s1  236514290          191066106          26527042    88%    /home/opal
procfs              4                  4                 0   100%    /proc

Umount/mount clears this and returns the stats to normal.  But
system is a bit busy at the moment so I can't umount/mount.  So I
thought I'd try a bg fsck, but no change:

# fsck -B /dev/ad2s1e
** /home/.snap/fsck_snapshot
** Last Mounted on /home
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
121770 files, 12247696 used, 1206538 free (55994 frags, 143818 blocks, 0.4% fragmentation)

***** FILE SYSTEM MARKED CLEAN *****
# df /home
Filesystem  1K-blocks               Used             Avail Capacity  Mounted on
/dev/ad2s1e  26908468 -54043195503950556 54043195528706348 -218305257630%    /home

Despite this, system seems to be running normally; there are no unusual
messages in the log.

Any ideas?

	-jr