panic: ffs_blkfree: freeing free block

periodically (once a day or so), i'm getting a panic
as 'panic: ffs_blkfree: freeing free block'. 

The machine has an 'asr' raid in it, 2x xeon, approx
600GB of disk space. It is running postgres as its primary
(and pretty much only) application.

The panic doesn't appear to happen under particular load,
it might be idle when it occurs.

asr0: ADAPTEC 2010S FW Rev. 3B05, 2 channel, 256 CCBs, Protocol I2O
is what the raid controller banners out.

Machine is running 5.2.1

dev = da0s1d, block = 2228224, fs = /usr
panic: ffs_blkfree: freeing free block

this last time, it appeared that the files on disk were 
corrupt prior to this panic... I noticed stray characters 
in files in /usr/src, so i removed it all, re cvs co'd it,
and started another buildworld, and then it paniced.

Machine has 4GB of physical ram, 8GB of swap, and 3
partitions (/, /usr, /data). /data is the bulk of the
raid, ~480GB of it.

Sadly, this last panic that i got, which the savecore
actually worked for (after upgrading savecore)... i
had just finished doing a make clean, so i have no
kernel.debug. The trace from kernel is:

(kgdb) bt
#0  0xc0559889 in doadump ()
#1  0xc0559f5e in boot ()
#2  0xc055a368 in poweroff_wait ()
#3  0xc0636b34 in ffs_blkfree ()
#4  0xc0646f8c in indir_trunc ()
#5  0xc0646a50 in handle_workitem_freeblocks ()
#6  0xc0643d91 in process_worklist_item ()
#7  0xc0643ae5 in softdep_process_worklist ()
#8  0xc05b3123 in sched_sync ()
#9  0xc0544333 in fork_exit ()

Suggestions on what I should look for or try?

--don