reproducible crash in ext2fs internals

Hi all.
My NetBSD-3.1-rc2 box reproducibly crashes somewhere in ext2fs internals.

Both Linux's and NetBSD's fsck.ext2 doesn't help.

Last things I see are

start=0, len=1,fs=/xxx
panic" ext2fs_alloccg: map corrupted
Stopped in pid 2645.1 (tee) at netbsd cpu Debugger+0x4 leave

bt command shows the following call stack: cpu_Debugger, panic,
ext2fs_mapsearch, ext2fs_alloccg, ext2fs_hashalloc, ext2fs_alloc,
ext2fs_balloc, ext2fs_gap_alloc, ufs_balloc_range, ext2fs_white

What kind of information I should provide to help you make a fix?
Section 15.4 of NetBSD guide says something strange ;-)
about backup and wd0b which I havn't got.

-- 
Best regards, Aleksey Cheusov.