Barring a bug (and  no such bug has been reported),  this can only happen
if the file is  edited by hand, or (under unix) if  the file is corrupted
after a  system crash  or the  like. Under  VM there  have been  a couple
instances  of people  putting the  LISTSERV code  on a  R/O disk  and the
migration procedure  failing to  erase a REXX  procedure that  had become
obsolete and did not  know about the new LIST format.  In general you get
unpredictable results  on VM if you  do not put  the code on the  A disk.
Some people do it because of security auditing requirements, but it's not
a supported mode. At the very least, it requires manual installation. The
auto-install method can't know what special  local steps it needs to take
to update  write protected code, besides  in most cases these  steps will
start with "log in to LISTADM  userid" or "call security auditor" or some
other action  a program can't  do. Anyway, this is  maybe 2 sites  in the
network, so I'm only mentioning it for the sake of completeness.
 
  Eric