disappearing files

[admin at prnet.org]

Hi,

I am using FreeBSD 8.2 with an UFS filesystem containing a jail. I do a
tar of the whole jail every night. Suddenly after a few weeks of
operation, tar complains about being unable to stat /usr/lib/libmilter.so.

>From inside the jail:

ls /usr/lib | grep "^libmilter.so$" returns "libmilter.so"
ls /usr/lib/libmilter.so returns "ls: libmilter.so: No such file or
directory"
ls -l /usr/lib > /dev/null returns "ls: libmilter.so: No such file or
directory"

Trying to access the file from the main system gives the same result.

A reboot returns everything to normal.

Exactly the same also appeared on another machine some time ago, the only
difference was that it appeared on another file.

Does anyone know what can cause this problem ?

Thanks in advance,
David Arendt