firewall code

firewall code n.  1. The code you put in a system (say, a
   telephone switch) to make sure that the users can't do any
   damage. Since users always want to be able to do everything but
   never want to suffer for any mistakes, the construction of a
   firewall is a question not only of defensive coding but also of
   interface presentation, so that users don't even get curious about
   those corners of a system where they can burn themselves.
   2. Any sanity check inserted to catch a {can't happen} error.
   Wise programmers often change code to fix a bug twice: once to fix
   the bug, and once to insert a firewall which would have arrested
   the bug before it did quite as much damage.