Someone jacked the network cable from this
machine. Replaced it, works fine.

I had a student try to log on to this computer
unsuccessfully.  I restarted the machine, and
tried with my own password to no avail.  The
student moved to the adjacent machine and logged
in just fine.  I tried moving the cursor into the
username field and then hitting the backspace key
(I've personally had difficulty logging onto a Mac
because there was an invisible space at the first
character of the username field.)  But that didn't
work either.  At this point I've tried everything
I know.