Sometimes a mv between directories will rename the file but leave it
in the same directory. Unfortunately it's not as reproducable as I'd
like, but here's the basic idea:

$ mkdir a b
$ touch a/c
$ while mv a/c b/d && mv b/d a/c; do 9 ls a b; done; 9 ls a b
a/c
mv: cannot stat `b/d': No such file or directory
a/d

 The second time around when I ran the loop it seemed to be working
consistently, then I ctrl-C'd it and now I have a/c *and* b/d, but
attempts to access a/c are met with "Bad file descriptor". After I
removed a/c and tried to recreate it I seem to have crashed u9fs.
 I can reproduce the problem with spfs's ufs. It took longer, and each
individual mv was quicker, but I don't know if these observations are
relevant or just circumstancial.

 Versions:
linux 2.6.22
coreutils 6.9
u9fs (non .u version, from plan9)
spfs/ufs ~20061225