file system on / grew over night?

[mmiller3@iupui.edu (Michael A. Miller)]

I came in this morning to find that the / filesystem on a machine
is at 100%. This is on a stable machine installed with one
partition as per the installer suggestions.  I've cleared off
enough space to be able to get log in and find that the disk
seems full: 

/dev/hda1              71G   65G  3.0G  96% /

But when I du to find out what caused it, I can only find 47 GB
in use.  So it seems that something happened to create 18 GB that
I cannot find.  Does anyone have suggestions as to how to trouble
shoot this?

Thanks, Mike