What does this error message mean:

ffs_update: bad indirect addr on entry (1): inode=3408 4194304/00400000
ip=f1501200 adr=f15012b0
inode vp: type VBLK, usecount 118, writecount 0, refcount 4, tag VT_UFS, ino
3383, on dev 24, 0

It first showed up with bsd-4444. It is reported during file accesses, in this
case it came when I shut down the machine (while unmounting /usr).
It happened with different inodes. The files to which these inodes correspond
are all perfectly readable. fsck does not report any problems.
I tried to locate the error message in the kernel sources, but with no luck.

So what does the whole thing mean?
Hope somebody can enlighten me,

Markus