Note that, the block that causes panic does not on the full partition...

FreeBSD ntucsp.csie.ntu.edu.tw 5.5-PRERELEASE FreeBSD 5.5-PRERELEASE #1: Thu Mar  9 10:36:18 CST 2006     root@ntucsp.csie.ntu.edu.tw:/usr/obj/usr/src/sys/NTUCSP  i386


May  2 17:59:14 ntucsp kernel: pid 498 (nfsd), uid 0 inumber 2897489 on /export/
b4: filesystem full
dev = da0s1f, block = 6546920, fs = /export/b3
panic: ffs_blkfree: freeing free block
cpuid = 2
KDB: stack backtrace:
kdb_backtrace(c068b55b,2,c0683c7b,e9f60ad0,5) at kdb_backtrace+0x2f
panic(c0683c7b,c23435a8,63e5e8,0,c23760d4) at panic+0x128
ffs_blkfree(c2376000,c23d0738,63e5e8,0,4000) at ffs_blkfree+0x3d2
indir_trunc(c3b10a00,18f9640,0,1,80c) at indir_trunc+0x30d
handle_workitem_freeblocks(c3b10a00,0,2,6,0) at handle_workitem_freeblocks+0x20e
process_worklist_item(0,0,44572e9e,0,0) at process_worklist_item+0x1e1
softdep_process_worklist(0,1e,c1f5c180,0,0) at softdep_process_worklist+0xcc
sched_sync(0,e9f60d38,c7d83,4890874,c55ff24) at sched_sync+0x5f2
fork_exit(c054a5c1,0,e9f60d38) at fork_exit+0x80
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xe9f60d6c, ebp = 0 ---
boot() called on cpu#0
Uptime: 13h32m35s
Dumping 1023 MB
 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240 256 272 288 304 320 336 352 368 384 400 416 432 448 464 480 496 512 528 544 560 576 592 608 624 640 656 672 688 704 720 736 752 768 784 800 816 832 848 864 880 896 912 928 944 960 976 992 1008
Dump complete
Automatic reboot in 1 seconds - press a key on the console to abort
Rebooting...
cpu_reset called on cpu#0