Terry,
 
  I've run into this same problem on my 957.  After a few minutes, the session
will go away.  I've also found that sending a :WARN to the session causes it
to log off.  I haven't seen anything that explains why, but I would assume that
something in the OS becomes aware that the session was orphaned at that time
and kills it.  No danger of crashing the system, either!
 
Bob Rose