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.
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.
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