panic: ffs_blkfree_cg: freeing free block

A panic occured while I was ``rm -rf''ing a large file&directory tree
(that I just created with untar) on an old drive that I have not used
for a long time. Unfortunately I'm not 100% sure that the filesystem
was clean when I mounted it today. Could that result in such a panic?

I don't have the intermediate object files for the kernel; now I'm
building the kernel again (from the appropriate, exact sources). That
shouldn't harm debugging, should it? Meanwhile, I'll take any debug
info requests, which I'll attempt to address shortly.