ZFS panic: existing znode

FreeBSD 8-CURRENT, built from sources around 27/02/2009:

FreeBSD foo.internal 8.0-CURRENT FreeBSD 8.0-CURRENT #5: Fri Apr 17 18:33:02 BST 2009 mwest_at_foo.internal:/usr/obj/usr/src/sys/DEBUGLOCK amd64

The system is AMD64, with 16GB of RAM, serving a few hundred clients via
NFS (v2 and v3) and Samba, from a 800GB ZFS pool; using hardware RAID
(aac controller), not RAID-Z.

Running a GENERIC kernel, but with the following options enabled:

options DEBUG_LOCKS
options DEBUG_VFS_LOCKS
options DIAGNOSTIC

I also have Jaakko Heinonen's patch to zfs_znode.c applied, from:
http://www.freebsd.org/cgi/query-pr.cgi?pr=132068

After almost a week of active usage, there was a system panic.

----------
panic: existing znode 0xffffff007dfd3d38 for dbuf 0xffffff00aeeb9620
cpuid = 2
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2a
panic() at panic+0x182
zfs_znode_dmu_init() at zfs_znode_dmu_init+0xb5
zfs_znode_alloc() at zfs_znode_alloc+0xa0
zfs_mknode() at zfs_mknode+0x205
zfs_freebsd_create() at zfs_freebsd_create+0x617
VOP_CREATE_APV() at VOP_CREATE_APV+0xb3
nfsrv_create() at nfsrv_create+0x909
nfssvc() at nfssvc+0x4af
syscall() at syscall+0x1e7
Xfast_syscall() at Xfast_syscall+0xab
--- syscall (155, FreeBSD ELF64, nfssvc), rip = 0x800695c4c, rsp = 0x7fffffffe8e8, rbp = 0 ---
----------

I see Kris Kennaway encountered a similar crash, but no one seems to
have replied:

  http://lists.freebsd.org/pipermail/freebsd-current/2009-January/002631.html

I was unfortunately not able to generate a crash dump.

Let me know if there's any further information I can provide which
might be useful.

Thanks,

Matthew