This probably happened to me under RH 6.1, but I had just received my 6.2 
copy, so I just did a fresh install rather than track down the problem.

The symptoms are that I was working just fine with X Window support 
working, shut down my system for the night, and when I rebooted the next 
morning I got a flickery display instead of the X Window signon screen.

After some digging, I determined that the cause of my X Window problem was 
that xfs was not starting successfully. After some more digging, I 
determined that the cause of xfs not starting successfully was that I was 
out of disk space on /tmp. Looking there, I found one humongous temp file.

Thinking back on what I did yesterday that might have been out-of-the-
ordinary, the only thing I can think of was that I attempted to do 
a "diff" of two huge text files. After quite a while, the diff exited with 
the message that I had run out of "memory". It didn't say anything about 
being out of disk space, but I conjecture that that's what it really 
meant, and that it did not clean up its temp file, leaving it there, 
leaving my root partition full, and preventing xfs from starting the next 
day.

I do not know whether this is a problem specific to diff or whether it is 
based upon a library routine used by diff and other programs.

Thanks!