Marcus J. Ranum quotes:

One person's "paranoia" is another person's "engineering redundancy."

As security or firewall administrators, we've got basically the same concerns [as plumbers]: the size of the pipe, the contents of the pipe, making sure the correct traffic is in the correct pipes, and keeping the pipes from splitting and leaking all over the place. Of course, like plumbers, when the pipes do leak, we're the ones responsible for cleaning up the mess, and we're the ones who come up smelling awful...