ffs_clusteralloc: allocated out of group on 5TB fs (5.3-Stable)

On a machine with a 5TB array I created a single partition via gpt and a
filesystem using the defaults from newfs.

All seemed well until I tried to fill the partition with data.  With "only"
870GB or so used utilities started reporting the disk as full. Shortly (a
few hours) after the machine panic'd.

During this time I had been trying to copy file via nfs and rsync onto the
array.  After several crashes I tried a simple "dd if=/dev/zero of=./file"
and after writing ~33GB the machine rebooted (the panic details below are
from this reboot)

Details follow:

# ls -l /fs/file
-rw-r--r--  1 root  wheel  35446063104 Nov 29 15:00 /fs/file
# gpt -r -v show /dev/da0
gpt show: /dev/da0: mediasize=5597405642752; sectorsize=512;
blocks=10932432896
        start         size  index  contents
            0            1         PMBR
            1            1         Pri GPT header
            2           32         Pri GPT table
           34  10932432829      1  GPT part - FreeBSD UFS/UFS2
  10932432863           32         Sec GPT table
  10932432895            1         Sec GPT header
# df -h /fs
Filesystem    Size    Used   Avail Capacity  Mounted on
/dev/da0p1    4.9T    897G    3.7T    19%    /fs
# df -ki /fs
Filesystem 1K-blocks       Used      Avail Capacity iused     ifree %iused
Mounted on
/dev/da0p1 5294245800 940234932 3930471204    19% 2190679 681971367    0%
/fs

Error messages before panic:
Nov 29 12:29:46 mach kernel: pid 6749 (bsdtar), uid 0 inumber 234766339 on
/fs: filesystem full
Nov 29 14:19:00 mach kernel: pid 34204 (pax), uid 0 inumber 505543685 on
/fs: filesystem full

Dumpfs header:
magic   19540119 (UFS2) time    Mon Nov 29 15:23:41 2004
superblock location     65536   id      [ 419208fe aae47fbc ]
ncg     29049   size    2733108207      blocks  2647122900
bsize   16384   shift   14      mask    0xffffc000
fsize   2048    shift   11      mask    0xfffff800
frag    8       shift   3       fsbtodb 2
minfree 8%      optim   time    symlinklen 120
maxbsize 16384  maxbpg  2048    maxcontig 8     contigsumsize 8
nbfree  272113785       ndir    267986  nifree  681971367       nffree 95154
bpg     11761   fpg     94088   ipg     23552
nindir  2048    inopb   64      maxfilesize     140806241583103
sbsize  2048    cgsize  16384   csaddr  3000    cssize  464896
sblkno  40      cblkno  48      iblkno  56      dblkno  3000
cgrotor 22824   fmod    0       ronly   0       clean   0
avgfpdir 64     avgfilesize 16384
flags   none
fsmnt   /fs
volname         swuid   0

info file contents:
Good dump found on device /dev/da1s1e
  Architecture: i386
  Architecture version: 1
  Dump length: 2147483648B (2048 MB)
  Blocksize: 512
  Dumptime: Mon Nov 29 15:01:10 2004
  Hostname: mach.example.com
  Versionstring: FreeBSD 5.3-STABLE #0: Fri Nov 12 13:05:18 UTC 2004
    root_at_mach.example.com:/usr/obj/usr/src/sys/SMP
  Panicstring: ffs_clusteralloc: allocated out of group
  Bounds: 0

Backtrace:
#0  0xc060ce82 in doadump ()
#1  0xc060d47b in boot ()
#2  0xc060d7a1 in panic ()
#3  0xc073445e in ffs_clusteralloc ()
#4  0xc0733680 in ffs_hashalloc ()
#5  0xc07329ea in ffs_reallocblks_ufs2 ()
#6  0xc073205a in ffs_reallocblks ()
#7  0xc0657e7c in cluster_write ()
#8  0xc074c350 in ffs_write ()
#9  0xc066bf10 in vn_write ()
#10 0xc062d624 in dofilewrite ()
#11 0xc062d4d1 in write ()
#12 0xc07bb07f in syscall ()
#13 0xc07a8a0f in Xint0x80_syscall ()

Kernel is the default SMP kernel and sources are from 28th Nov 2004.

I assume a proper debug kernel with symbols would be more helpful, but I
just wanted to know if this was a known bug - or something I have done
wrong, first.
Other than that is there anything else I can provide to help find this bug?

Thanks

Henry